Expression of nicotinic acetylcholine receptors in Alzheimer's disease: postmortem investigations and experimental approaches

Intracellular accumulation of beta-amyloid(1-42) in neurons is facilitated by the alpha 7 nicotinic acetylcholine receptor in Alzheimer's disease

Amyloid beta(1-42), a major component of amyloid plaques, binds with exceptionally high affinity to the alpha 7 nicotinic acetylcholine receptor and accumulates intracellularly in neurons of Alzheimer's disease brains. In this study, we investigated the possibility that this binding plays a key role in facilitating intraneuronal accumulation of amyloid beta(1-42). Consecutive section immunohistochemistry and digital imaging were used to reveal the spatial relationship between amyloid beta(1-42) and the alpha 7 receptor in affected neurons of Alzheimer's disease brains. Results showed that neurons containing substantial intracellular accumulations of amyloid beta(1-42) invariably express relatively high levels of the alpha 7 receptor. Furthermore, this receptor is highly co-localized with amyloid beta(1-42) within neurons of Alzheimer's disease brains. To experimentally test the possibility that the binding interaction between exogenous amyloid beta(1-42) and the alpha 7 receptor facilitates internalization and intracellular accumulation of amyloid beta(1-42) in Alzheimer's disease brains, we studied the fate of exogenous amyloid beta(1-42) and its interaction with the alpha 7 receptor in vitro using cultured, transfected neuroblastoma cells that express elevated levels of this receptor. Transfected cells exhibited rapid binding, internalization and accumulation of exogenous amyloid beta(1-42), but not amyloid beta(1-40). Furthermore, the rate and extent of amyloid beta(1-42) internalization was related directly to the alpha 7 receptor protein level, since (1) the rate of amyloid beta(1-42) accumulation was much lower in untransfected cells that express much lower levels of this receptor and (2) internalization was effectively blocked by alpha-bungarotoxin, an alpha 7 receptor antagonist. As in neurons of Alzheimer's disease brains, the alpha 7 receptor in transfected cells was precisely co-localized with amyloid beta(1-42) in prominent intracellular aggregates. Internalization of amyloid beta(1-42) in transfected cells was blocked by phenylarsine oxide, an inhibitor of endocytosis. We suggest that the intraneuronal accumulation of amyloid beta(1-42) in Alzheimer's disease brains occurs predominantly in neurons that express the alpha 7 receptor. In addition, internalization of amyloid beta(1-42) may be facilitated by the high-affinity binding of amyloid beta(1-42) to the alpha 7 receptor on neuronal cell surfaces, followed by endocytosis of the resulting complex. This provides a plausible explanation for the selective vulnerability of neurons expressing the alpha 7 receptor in Alzheimer's disease brains and for the fact that amyloid beta(1-42) is the dominant amyloid beta peptide species in intracellular accumulations and amyloid plaques.

Mutations of the neuronal nicotinic acetylcholine receptors and their association with ADNFLE

Elucidating the origin of epileptic seizures represents one of the many ways by which today's scientists are approaching this devastating neurological disorder. Although epilepsies have several different origins ranging from head trauma to genetically transmissible affections, common neuronal network dysfunction can be recognised between these many forms of the disease. Thus, understanding the basic mechanisms underlying some genetically transmissible epilepsies should bring new and important knowledge that is readily applicable to other types of epilepsies. In this work we review our current knowledge of one genetically transmissible form of nocturnal epilepsy, the ADNFLE. In the light of the most recent findings obtained on five mutants of the neuronal nicotinic acetylcholine receptors associated with ADNFLE, we discuss the effects of these spontaneous genome alterations on the receptor function. The only common trait identified so far between these mutant receptors is an increase in acetylcholine sensitivity. Based on our understanding of the receptor distribution in the different brain areas, their development and the neuronal network circuitry, we hypothesise that increased acetylcholine sensitivity causes an unbalance in the fine tuning of the cortico-reticular thalamic and thalamo-cortical loops. In addition, we illustrate how spontaneous mutations in the gene coding for a receptor provoke a change in its pharmacological profile and thereby might account for the inter-individual therapeutic sensitivity.

Nicotinic cholinergic modulation: galantamine as a prototype

Nicotinic acetylcholine receptor pharmacology is becoming increasingly important in the clinical symptomatology of neurodegenerative diseases in general and of cognitive and behavioral aspects in particular. In addition, the concept of allosteric modulation of nicotinic acetylcholine receptors has become a research focus for the development of therapeutic agents. In this review the scientific evidence for changes in nicotinic acetylcholine receptors in Alzheimer's disease is described. Within this context, the pharmacology of galantamine, a recently approved drug for cognition enhancement in Alzheimer's disease, is reviewed along with preclinical studies of its efficacy on learning and memory. Galantamine modestly inhibits acetylcholinesterase and has an allosteric potentiating ligand effect at nicotinic receptors. The data collected in this review suggest that the unique combination of acetylcholinesterase inhibition and nicotinic acetylcholine receptor modulation offers potentially significant benefits over acetylcholinesterase inhibition alone in facilitating acetylcholine neurotransmission.

Histochemical study of acute and chronic intraperitoneal nicotine effects on several glycolytic and Krebs cycle dehydrogenase activities in the frontoparietal cortex and subcortical nuclei of the rat brain

The effects of nicotine on the activity of different dehydrogenases in frontoparietal regions and subcortical nuclei of the rat brain have been studied using histochemical methods. Nicotine sulphate was intraperitoneally administered in acute (4 mg/kg/day x 3 days) or chronic (ALZET osmotic pump providing 2 mg/kg/day x 15 days) doses. The enzymes analyzed were glyceraldehyde-3-phosphate, lactate, malate and succinate dehydrogenases (gly3PDH, LDH, MDH, and SDH, respectively). The results demonstrate that chronic as well as acute administration of nicotine produced strong increases in all these enzymatic activities in the superior layers (I, II and III) of the frontoparietal cortex (cingulate, motor and somatosensory regions); but high increases were not seen in the deeper layers of the cortex or in the subcortical nuclei (substantia nigra, caudate-putamen, nucleus accumbens or nucleus basalis magnocellularis). These hyperactivities were produced in brain regions with normally low enzymatic activity (cortex), but not in those with great intensity (subcortical nuclei). The results are in rough agreement with previous reports on nicotine-induced increases in glucose utilization, gly3PDH genic expression and neuronal hyperactivity in the brain cortex; but significant discrepancies between the cortical enzymatic maps and those obtained both in these studies and others on nicotine(N)-receptor localization have been appreciated. The results support the hypothesis that nicotinic cholinergic drugs can have metabolic, long-lasting stimulant effects on cortical neurons at specific points (probably layer III pyramidal cells and structures with alpha7-N-receptors) of the cortical circuits that could be of great interest in improving altered cognitive functions that are present in Alzheimer disease, as well as in other less severe mental disturbances. Mitochondrial hyperfunction should also be evaluated as a possible side-effect (as an oxidative stress inductor) of these kinds of drugs.

Nucleus basalis Meynert lesions and the expression of nicotinic acetylcholine receptor proteins in the rat frontal cerebral cortex

An important feature of cholinergic dysfunction in Alzheimer's disease (AD) is the degenerative loss of magnocellular cholinergic neurons in the basal nucleus of Meynert. In search for suitable animal models of Alzheimer dementia, rats with lesioned basal nuclei rats have been shown to display learning and memory disturbances. We here report on the quantitative assessment of the expression of the nicotinic acetylcholine receptor alpha4 protein in the rat frontal cerebral cortex following a unilateral lesion of the basal nucleus. Cortical alpha4 isoform expression shows a significant increase on the lesioned vs. the non-lesioned control side 1 week after lesioning. By contrast, no differences were observed 4 weeks after lesioning. In consideration of these results basal nucleus lesions appear as a questionable model of AD which in contrast to the present findings shows a decrease of cortical alpha4 nicotinic acetylcholine receptor protein expression.

All three PACSIN isoforms bind to endocytic proteins and inhibit endocytosis

The PACSINs are a family of cytoplasmic phosphoproteins that play a role in vesicle formation and transport. We report the cloning and cDNA sequencing of PACSIN 3 and the analysis of all three PACSIN isoforms with regard to tissue distribution, ligand binding properties and influence on endocytosis. PACSIN 3 differs from the other family members in having a short proline-rich region and lacking asparagine-proline-phenylalanine motifs. In contrast to the neurospecific PACSIN 1 and the ubiquitously expressed PACSIN 2, PACSIN 3 is mainly detected in lung and muscle tissues. All isoforms potentially oligomerize and bind to dynamin, synaptojanin 1 and N-WASP via their Src homology 3 domains. The PACSIN proteins colocalize with dynamin, but not with clathrin, implying a specific role with a distinct subpopulation of dynamin at defined cellular sites. Transferrin endocytosis is blocked in a dose-dependent manner in cells overexpressing the PACSIN variants, but the inhibitory effect can be abolished by mutating specific amino acid residues in the Src homology 3 domains. These characteristics of the PACSIN protein family suggest a general function in recruitment of the interacting proteins to sites of endocytosis.