Pathogenesis of Alzheimer's disease--beyond the cholinergic hypothesis: discussion paper

Targeting Alzheimer's disease by investigating previously unexplored chemical space surrounding the cholinesterase inhibitor donepezil

A series of twenty seven acetylcholinesterase inhibitors, as potential agents for the treatment of Alzheimer's disease, were designed and synthesised based upon previously unexplored chemical space surrounding the molecular skeleton of the drug donepezil, which is currently used for the management of mild to severe Alzheimer's disease. Two series of analogues were prepared, the first looking at the replacement of the piperidine ring in donepezil with different sized saturated N-containing ring systems and the second looking at the introduction of different linkers between the indanone and piperidine rings in donepezil. The most active analogue 5,6-dimethoxy-1-oxo-2,3-dihydro-1H-inden-2-yl 1-benzylpiperidine-4-carboxylate (67) afforded an in vitro IC₅₀ value of 0.03 ± 0.07 μM against acetylcholinesterase with no cytotoxicity observed (IC₅₀ of >100 μM, SH-SY5Y cell line). In comparison donepezil had an IC₅₀ of 0.05 ± 0.06 μM and an observed cytotoxicity IC₅₀ of 15.54 ± 1.12 μM. Molecular modelling showed a strong correlation between activity and in silico binding in the active site of acetylcholinesterase.

The 3D-QSAR study of 110 diverse, dual binding, acetylcholinesterase inhibitors based on alignment independent descriptors (GRIND-2). The effects of conformation on predictive power and interpretability of the models

The 3D-QSAR analysis based on alignment independent descriptors (GRIND-2) was performed on the set of 110 structurally diverse, dual binding AChE reversible inhibitors. Three separate models were built, based on different conformations, generated following next criteria: (i) minimum energy conformations, (ii) conformation most similar to the co-crystalized ligand conformation, and (iii) docked conformation. We found that regardless on conformation used, all the three models had good statistic and predictivity. The models revealed the importance of protonated pyridine nitrogen of tacrine moiety for anti AChE activity, and recognized HBA and HBD interactions as highly important for the potency. This was revealed by the variables associated with protonated pyridinium nitrogen, and the two amino groups of the linker. MIFs calculated with the N1 (pyridinium nitrogen) and the DRY GRID probes in the AChE active site enabled us to establish the relationship between amino acid residues within AChE active site and the variables having high impact on models. External predictive power of the models was tested on the set of 40 AChE reversible inhibitors, most of them structurally different from the training set. Some of those compounds were tested on the different enzyme source. We found that external predictivity was highly sensitive on conformations used. Model based on docked conformations had superior predictive ability, emphasizing the need for the employment of conformations built by taking into account geometrical restrictions of AChE active site gorge.

Comparison of the cholinergic system in neocortical field Ep in cats with strong and weak cognitive abilities

Comparative analysis of the cholinergic system of the neocortex, consisting of the terminals of neurons from the magnocellular basal nuclei and intracortical neurons, in cats with strong and weak abilities to solve abstraction and generalization tasks was performed by isolating subfractions of synaptic membranes and synaptoplasm from "light" (C) and "heavy" (D) synaptosomes from associative field Ep and measuring choline acetyltransferase and acetylcholinesterase activities, protein content, and sulfhydryl group concentrations. These experiments showed that all measures were significantly lower in subfractions from C synaptosomes from cats with strong cognitive abilities. This leads to the conclusion that relatively small numbers of cholinergic synapses form in field Ep of the brains of cats with strong cognitive abilities, while their location in the C fraction demonstrates that they correspond to neurons of the magnocellular basal nuclei. The possible physiological significance of the "deficiency" of cholinergic inputs in field Ep from these nuclei as a correlate of the animal's cognitive ability is discussed. The D subfractions from able cats had significantly higher acetylcholinesterase activity, while choline acetyltransferase activity was not different; this identifies differences between groups of animals in the organization of non-cholinergic acetylcholinesterase-containing synapses in field Ep.

Destruction of the cholinergic basal forebrain using immunotoxin to rat NGF receptor: modeling the cholinergic degeneration of Alzheimer's disease

Degeneration of cholinergic neurons in the basal forebrain (CBF) is a prominent neuropathological feature of Alzheimer's disease and is thought responsible for some cognitive deficits seen in patients. An animal model of pure CBF degeneration would be valuable for analysis of the function of these neurons and testing therapeutic strategies. CBF neurons express receptors for nerve growth factor. In order to selectively destroy these neurons, we developed an immunotoxin using monoclonal antibody (192 IgG) to rat NGF receptor (p75NGFr) armed with the ribosome inactivating protein, saporin. In vitro 192-saporin was highly toxic to neurons expressing p75NGFr. Intraventricular injections of 192-saporin destroyed the CBF and impaired passive avoidance learning. These results indicate that 192-saporin treated rats can be used to model a key feature of Alzheimer's disease and that anti-neuronal immunotoxins are a powerful approach to selective neural lesioning.

Intermediary metabolism disturbance in AD/SDAT and its relation to molecular events

1. Early-onset dementia of Alzheimer type (EODAT; AD) and late-onset dementia of Alzheimer type (LODAT; SDAT) are heterogenous in origin. 2. A common superordinate pathobiochemical principle in the etiopathogenesis of both types of dementia is neuronal energy failure with subsequent abnormalities in cellular Ca2+ homeostasis and glucose-related amino acid metabolism. 3. These metabolic abnormalities are assumed to occur first at axodendritic terminals of the acetylcholinergic-glutamatergic circuit and to cause morphological damage at synaptic sites. 4. Metabolic stress and structural damage at synaptic sites may induce enhanced formation of APP and its cleavage product amyloid. 5. Energy-metabolism related abnormalities along with functional and structural changes at synaptic sites of the acetylcholinergic-glutamatergic circuit may precede the formation of amyloid in DAT brain.

The role of cortical connectivity in Alzheimer's disease pathogenesis: a review and model system

Here we review current evidence in support of the cortical disconnection/cortical connectivity model of Alzheimer disease (AD) pathogenesis, a model which predicts that one of the first events in AD is damage to the entorhinal cortex and/or subiculum resulting in the disconnection of the hippocampal formation and neocortex, and the subsequent progression of the disease in a stepwise fashion along cortico-cortical connections. Much of the evidence for this model has been obtained from studies involving the limbic system where investigators have demonstrated a precise correspondence between established patterns of connectivity and the degenerative changes associated with AD. In addition, some studies of the distribution of neuritic plaques (NP) and neuro-fibrillary tangles (NFT) in the neocortex and subcortical structures have yielded corroborative data. The validity of the cortical disconnection/connectivity model in the neocortex remains to be established or refuted. We propose that testing of this model can be accomplished with systematic studies of the laminar and regional distribution of NP and NFT in a series of sequentially interconnected cytoarchitectural regions that also form part of two functional hierarchies--the paralimbic and occipitotemporal visual systems. To adequately control for variation between brains affected by AD, it is imperative that such studies be conducted in a large but varied population of AD cases exhibiting differences in several variables, including clinical and/or neuropathological severity of the disease, temporal duration of the disease, and clinical/neuropsychological profile. We believe that further understanding of the relationship between characteristic AD pathology and intrinsic anatomico-functional circuits will contribute not only to our comprehension of AD pathogenesis but also to our general knowledge of the human brain.

Bilateral olfactory dysfunction in early stage treated and untreated idiopathic Parkinson's disease

Decreased olfactory function is among the first signs of idiopathic Parkinson's disease (PD). Whether such dysfunction is present to the same degree on both sides of the nose, however, is unknown. Furthermore, whether the deficit results from or is influenced by anti-Parkinsonian medications has not been definitely established. Odour identification ability was evaluated on the left and right sides of the nose in 20 early-stage untreated PD patients, 20 early-stage treated PD patients, and 20 controls. In all cases, the PD related olfactory dysfunction was bilateral and no difference was observed between the test scores of patients taking or not taking drugs for PD. Although asymmetries of unsystematic direction were present in the test scores of some PD patients, similar asymmetries were observed in the controls and the asymmetries were not related to the side of the major motor dysfunction. As in earlier work, no relation was present between the olfactory test scores and the degree of tremor, rigidity, bradykinesia, or gait disturbance at the time of testing. These findings indicate that the olfactory dysfunction of early stage PD is robust, typically of the same general magnitude on both sides of the nose, and uninfluenced by anti-Parkinsonian medications.

Olfaction and psychiatry

Recent clinical studies have identified significant olfactory deficits in several neuropsychiatric disorders, notably Alzheimer's disease and Parkinson's disease. These have correlated with neurochemical and neuropathological studies of the olfactory system. The presence of a specific sensory deficit may be related to the localisation of pathology within the brain. There is a need for incorporation of olfactory testing into routine clinical examination.

Influence of age and age-related diseases on olfactory function

It is clear from this review that olfactory function is markedly altered in old age and in a number of age-related diseases. The deficits appear to be rather general and detectable by several types of olfactory tests. Considerable interindividual variability exists, however, and the physiologic bases of these changes are not clear. In many healthy elderly persons, smell loss appears to occur as a result of one or more causes, including viral insult, cumulated exposure to toxic fumes, head trauma, and calcification of the cribriform plate. Several reviews have appeared suggesting that the olfactory system may be a center of primary involvement in AD. Of particular interest is the hypothesis that environmental agents (related etiologically to the disease process) pass into the central nervous system via the highly active transport mechanisms of the olfactory receptors. This latter notion, although attractive, must be viewed conservatively, as it is possible that the olfactory pathways are simply selectively vulnerable to destruction by various disease processes. This may explain why Huntington's chorea and multiinfarct dementia, in addition to AD and PD, are associated with alterations in smell function. Although it is tempting to assume, as have authors such as Koss et al., that alterations in threshold function reflect peripheral olfactory dysfunction and that alterations in odor identification and other more demanding tasks reflect central olfactory dysfunction, there is little empirical support for such a simple dichotomy. Despite the fact that a peripheral/central distinction is useful in clinical audiology (where threshold loss is commonly associated with CN VIII pathology), an evaluation of the utility of this distinction in olfaction requires further research. The limited data suggest that both identification and detection deficits commonly arise from damage to the olfactory epithelium, even though identification deficits unassociated with detection deficits may occur in some central brain disorders. It is apparent from the studies reviewed in this chapter that considerable progress has been made during the last decade in elucidating the nature and prevalence of olfactory disturbances in elderly patients, as well as in patients with dementia-related diseases.(ABSTRACT TRUNCATED AT 400 WORDS)

The natural history of mental disorder in old age. A long-term study

In a prospective study, 153 consecutive new referrals to a psychogeriatrician in Oxfordshire in 1973 were followed up for 15 years. The percentage of the over-65 population at risk was 0.27 and, of those aged over 80, 0.6. Alzheimer's dementia and depressive illness comprised over two-thirds of referrals. A quarter became permanent admissions, half of them with Alzheimer's dementia, 19% of whom were alive five years later. Depressive illness, at ten years, had a mortality 1.2-1.6 times that of the population at risk. The distinction between Alzheimer's dementia and depressive illness, based on their natural history and causes of death, was reinforced.