Current Alzheimer Research - Volume 6, Issue 2, 2009

Although significant accomplishments have been made in research to understand, diagnose and treat Alzheimer's disease (AD) and its prequel, mild cognitive impairment, over the last two decades, a huge amount more remains to be achieved to impact this incurable, terminal disease that afflicts an estimated 26.6 million people worldwide. Increasing evidence indicates that early diagnosis will be fundamental to maximizing treatment benefits. Moreover, mechanistically- based, hypothesis-driven treatment strategies are now emerging to hopefully spearhead future therapy. The crossfertilization of ideas from multiple disciplines will prove key to optimize strategies and translate them to meaningful clinical utility, and forms the basis of the current issue focused on “Advances in Alzheimer therapy”.

Cholinesterase inhibitors (ChEIs) were introduced in the therapy of Alzheimer Disease (AD) in the nineteen nineties with great expectations. The hopes and large interest raised by these drugs are well demonstrated by 12,000 references listed by PubMed under ‘ChEI’ for 1995-2007. The list is reduced to 2500 if we confine ourselves to ‘ChEIs and dementia’. Of them, about 500 were published in the last two years. Whereas an increase in brain acetylcholine and an improvement of cognitive deficits have been consistently demonstrated in animal models of AD, from aging rats to transgenic mice, the clinical effectiveness of ChEIs has been and is still a matter of contrasting opinions. These range from the negative conclusions of the AD2000 trial on donepezil, claiming that it is not cost effective, with benefits below a minimally relevant threshold, to the NICE appraisal of 2007 declaring that donepezil, rivastigmine, galantamine are efficacious for mild to moderate AD, irrespective of their different selectivity for acetyl- (AChE) and butyrylcholinesterase (BuChE). The possibility that ChEIs may exert their effects through mechanisms beyond cholinesterase inhibition has been envisaged. However, according to the information presented in this review, the “classical” ChEIs, donepezil, rivastigmine and galantamine, show no pharmacological actions beyond cholinesterase inhibition which may play an important role in their therapeutic efficacy. The diverging opinions on clinical efficacy do not discourage from developing new ChEIs, and particularly the so called multifunctional ChEIs. They represent the future of the cholinergic therapy for AD but other indications for these drugs may be considered, including vascular dementia, mild cognitive impairment, and the ethically sensitive improvement of memory and learning in healthy subjects.

Clinical trials of cholinesterase inhibitor (ChEI) drugs, although generally reporting only minimal improvements in patients with Alzheimer's disease (AD), indicate that a subgroup of patients may respond substantially to treatment. This study aimed to assess the clinically variable ChEI treatment effects in a group of patients with mild AD using a semantic association and an N-back light working memory activation paradigm. Twenty-six patients with probable mild AD treated with a ChEI for 20 weeks were retrospectively divided into responders and non-responders. Patients were classified as responders if their Clinician's Interview Based Impression of Change (CIBIC - Plus) score was four or less and if they had an increase of at least two points on the MMSE. These criteria resulted in two subgroups comprising nine responders and seventeen non responders. Nine healthy elderly age-matched controls were also recruited as a comparison group. ChEI treatment was accompanied by significant modulation of task induced activation increases in both fMRI tasks in AD responders. The effect of ChEI response was in effect a restoration of regional brain function in the same areas used by elderly controls when performing these tasks. In non-responders decrements in task related activation were observed and over time activation patterns appeared less like the elderly controls. Screening semantic fluency scores correlated negatively with activation increases at retest. In the paper, a tentative explanation is offered of why subgroups of patients with a similar clinical diagnosis and level of clinical severity show a different physiological response to ChE inhibition.

Cholinergic deficit is a cardinal feature of Alzheimer's disease, and cholinesterase inhibitors represent one of the most prominent means of mitigating this dysfunction. Cholinesterase inhibitors provide mild symptomatic relief, although they lose their efficacy over time most likely because they are not disease-modifying agents. An alternative strategy for restoring cholinergic function and attenuating the cognitive decline involves acting on the receptors on which acetylcholine acts. Stimulation of muscarinic acetylcholine receptors and in particular the M1 subtype has been shown to have a beneficial effect in restoring cognition in patients with Alzheimer's disease and in attenuating Aβ and tau pathology in different animal models. In this review, we discuss the role of M1 agonists as a potential disease-modifying therapy for Alzheimer's disease.

Background: Accumulation of beta-amyloid peptide (Aβ) in the brain is a primary influence driving Alzheimer's disease (AD) pathogenesis. The disease process, including formation of neurofibrillary tangles containing tau protein, is proposed to result from an imbalance between production and clearance of Aβ. A major therapeutic strategy for AD should be to decrease deposition of Aβ by the inhibition of its production and the facilitation of its degradation. Hence, the primary aim of this study was to investigate effects of GEPT, a combination of herbal extracts, on Aβ levels, β- and γ-secretases substrate (BACE1 and PS1, respectively) associated with production of Aβ, and insulin-degrading enzyme (IDE) and neprilysin (NEP) related to degradation of Aβ in the brain. Methods: Three-month-old-male APPV717I mice were randomly divided into five groups (n=6 per group): (i) APP mice alone were given distilled water, (ii) APP donepezil mice were treated with donepezil (0.92mg/kg/d), and (iii-v) APP mice treated with GEPT low dose (0.75g/kg/d), middle dose (1.5g/kg/d), and large dose (3.0g/kg/d) for 8 months. Three-month-old-male C57BL/6J mice (n=6) for vehicle were given distilled water for 8 months. Immunohistochemistry and Western blot analysis were used in determining amyloid precursor protein (APP), Aβ1-42, BACE1, PS1, IDE and NEP in hippocampal CA1 region and hippocampal tissue homogenates. Results: Expression level of Aβ1-42 in the large GEPT dose was significantly lower than those in APP alone or APP treated with donepezil, and decreased to the level of vehicle mice. Similarly, a ratio calculated from the densitometric measures of Aβ1-42 protein/β-actin in the large dose also was significantly lower than those in APP mice alone or APP mice treated with donepezil, and even reduced to the level of vehicle mice. Expression of PS1 in the large GEPT dose was significantly lower than that of APP mice alone and decreased to those in vehicle mice as well. A decreased level of BACE1 appeared, respectively, in APP mice treated with the large GEPT dose or donepezil but was still much greater than the level of vehicle mice. In contrast, NEP and IDE showed a significantly higher expression in APP mice treated with either the large dose or the middle dose of GEPT compared to APP mice alone or donepezil, and were even increased in level compared to vehicle mice. Conclusion: The combination of GEPT extracts can reduce levels of endogenous Aβ peptide in APPV717I transgenic mice through the inhibition of PS1 activity rather than BACE1 and the promotion of IDE and NEP activity. Lower-expression of PS1 and over-expression of IDE or NEP may be helpful in potentially lowering brain Aβ levels in subjects with AD, and hence GEPT appears to offer potential that should be explored in AD.

Oligomer Aβ is the term utilized for multimeric but non-fibrillar forms of amyloid β-protein (Aβ). The most prominent property of oligomer Aβ is considered to be its solubility and structure. Here, we examined the histochemical localization of oligomer Aβ in AD brains. At present, little information is available on the structure and function of cerebral oligomer Aβ. We therefore studied the molecular localization of oligomer Aβ using a newly generated polyclonal mouse antisera against a variant Aβ with a deletion mutation of the 22nd glutamate that we found recently in a patient with familial Alzheimer's disease. Intracellular as well as extracellular oligomer Aβ are herein discussed to define their structure and pathological roles in disease.

Objectives: Microglial overactivation, which is secondary to abnormalities of amyloid-beta peptide (Aβ) and tau proteins in the pathogenic cascade leading to onset of Alzheimer's disease (AD), accelerates tau pathology, according to our recent observations using mouse models of tauopathies, and this positive feedback results in formation of a vicious cycle between upstream and downstream processes, potentially hampering effective suppression of the entire cascade by anti-amyloid treatments. This motivates our present work aimed at dual monitoring of amyloidosis and microgliosis in living animal models of AD, toward therapeutic regulation of these two processes capable of halting the self-perpetuating cycle. Methods: Transgenic mice expressing mutant amyloid precursor protein (APP23 mice) was examined by highresolution positron emission tomography (PET) after administration of amyloid probe, Pittsburg Compound B (PIB) synthesized with high specific radioactivity (SA). Microglial activation in these mice was also imaged by PET and specific tracer, [18F]fluoroethyl-DAA1106. Results: Progressive amyloidosis in APP23 mice was visualized by PET and high-SA PIB. In vitro assays revealed preferential binding of PIB to N-terminally modified Aβ, AβN3pE. As levels of this Aβ subspecies in model mice are lower than those in AD patients, our findings plausibly explain advantages of high-SA tracers in sensitive detection of mouse amyloid. Near-simultaneous monitoring of amyloid removal and microgliosis in APP23 mice following injection of anti-Aβ antibody demonstrated positive correlation between levels of initially existing amyloid and antibody-induced microglial activation, suggesting the possibility of microglial overactivation in immunotherapy for subjects with abundant amyloid. Conclusions: The present animal imaging system would substantially facilitate establishment of a safe and effective therapeutic strategy targeting multiple key processes in the AD pathogenesis.

Background: Immunization of patients with Alzheimer's disease (AD) with synthetic amyloid-β peptide (Aβ42) (AN1792) was previously studied in a randomized, double-blind, placebo-controlled phase 2a clinical trial, Study AN1792(QS-21)-201. Treatment was discontinued following reports of encephalitis. One year follow-up revealed that AN1792 antibody responders showed improvements in cognitive measures as assessed by the neuropsychological test battery (NTB) and a decrease in brain volume compared with placebo. Methods: A follow-up study, Study AN1792(QS-21)-251, was conducted to assess the long-term functional, psychometric, neuroimaging, and safety outcomes of patients from the phase 2a study 4.6 years after immunization with AN1792. The results were analyzed by comparing patients originally identified as antibody responders in the AN1792 phase 2a study with placebo-treated patients. Results: One hundred and fifty-nine patients/caregivers (30 placebo; 129 AN1792) participated in this follow-up study. Of the 129 AN1792-treated patients, 25 were classified in the phase 2a study as antibody responders (anti-AN1792 titers ®1:2,200 at any time after the first injection). Low but detectable, sustained anti- AN1792 titers were found in 17 of 19 samples obtained from patients classified as antibody responders in the phase 2a study. No detectable anti-AN1792 antibodies were found in patients not classified as antibody responders in the phase 2a study. Significantly less decline was observed on the Disability Assessment for Dementia scale among antibody responders than placebo-treated patients (p=0.015) after 4.6 years. Significant differences in favor of responders were also observed on the Dependence Scale (p=0.033). Of the small number of patients who underwent a follow-up MRI, antibody responders showed similar brain volume loss during the follow-up period subsequent to the AN1792 phase 2a study compared with placebo-treated patients. Conclusions: Approximately 4.6 years after immunization with AN1792, patients defined as responders in the phase 2a study maintained low but detectable, sustained anti-AN1792 antibody titers and demonstrated significantly reduced functional decline compared with placebo-treated patients. Brain volume loss in antibody responders was not significantly different from placebo-treated patients approximately 3.6 years from the end of the original study. No further cases of encephalitis were noted. These data support the hypothesis that Aβ immunotherapy may have long-term functional benefits.

Alzheimer's disease is a neurodegenerative disorder characterized by amyloid deposits and neurofibrillary tangles. Cholinergic dysfunction is also a main pathological feature of the disease. Nevertheless, the links between cholinergic dysfunction and neuropathological hallmarks of Alzheimer's are still unknown. In the present study, we aimed to further investigate Tau aggregation in cholinergic systems, in a Tau transgenic mouse model. THY-Tau22 mice have recently been described as a novel model of Alzheimer-like Tau pathology without motor deficits. This strain presents an age-dependent development of Tau pathology leading to synaptic dysfunctions as well as learning and memory impairments. In the present work, we observed that Tau pathology differentially affects cerebral structures. Interestingly, early Tau pathology was observed in both hippocampus and basal forebrain. Moreover, some morphological as well as functional alterations of the septohippocampal pathway suggest a disconnection between these two key brain regions in Alzheimer's disease. Finally, these data suggest that Tau pathology may participate in cholinergic degeneration.

Nerve growth factor (NGF) deficits are linked to Alzheimer's Disease (AD), due to the role of NGF on basal forebrain cholinergic neurons (BFCN). We have further established that a disequilibrium in NGF signaling and/or processing from its precursor proNGF is also directly and causally related to the aberrant activation of an amyloidogenic route to neurodegeneration. The therapeutic potential of using human NGF to provide a long-lasting cholinergic trophic support, thereby preventing or slowing cognitive decline in AD patients, has therefore a strong rationale. However, a simple and practical means of delivering NGF to the brain in a safe and long-term manner, limiting the undesired adverse effects of NGF in activating nociceptive responses, has represented a significant challenge. For this reason, pilot clinical studies have been performed so far with invasive approaches requiring neurosurgery. We obtained a proof of principle, in neurodegeneration animal models, of an alternative, non-invasive delivery of NGF through an intranasal route, which facilitates access of NGF to the central nervous system (CNS), while minimizing the biodistribution of NGF to compartments where it activates undesired effects, such as pain. The ideal NGF product for a non invasive NGF-based therapy would be a recombinant NGF that, while exhibiting an identical biological activity to that of human NGF, can be traced, against the endogenous NGF, in order to optimize the therapeutical dose range and meet the required therapeutic window. We describe an engineered mutein of hNGF, hNGF-61, that is selectively recognized, against endogenous NGF, by a specific antibody. hNGF-61 mutein has an identical potency and bioactivity profile as hNGF, in vitro and in vivo. Moreover, hNGF-61 and hNGF are equally effective in rescuing the behavioral and neurodegenerative phenotype in adult and aged AD11 anti-NGF mice. Finally, we demonstrated that intranasally delivered hNGF-61 is significantly more effective than ocularly applied hNGF- 61, to determine phenotypic rescue in AD11 mice. The development of hNGF-61 towards clinical applications in AD patients is under way.

The most common animal models currently used for Alzheimer disease (AD) research are transgenic mice that express a mutant form of human Aβ precursor protein (APP) and/or some of the enzymes implicated in their metabolic processing. However, these transgenic mice carry their own APP and APP-processing enzymes, which may interfere in the production of different amyloid-beta (Aβ) peptides encoded by the human transgenes. Additionally, the genetic backgrounds of the different transgenic mice are a possible confounding factor with regard to crucial aspects of AD that they may (or may not) reproduce. Thus, although the usefulness of transgenic mice is undisputed, we hypothesized that additional relevant information on the physiopathology of AD could be obtained from other natural non-transgenic models. We have analyzed the chick embryo and the dog, which may be better experimental models because their enzymatic machinery for processing APP is almost identical to that of humans. The chick embryo is extremely easy to access and manipulate. It could be an advantageous natural model in which to study the cell biology and developmental function of APP and a potential assay system for drugs that regulate APP processing. The dog suffers from an age-related syndrome of cognitive dysfunction that naturally reproduces key aspects of AD including Aβ cortical pathology, neuronal degeneration and learning and memory disabilities. However, dense core neuritic plaques and neurofibrillary tangles have not been consistently demonstrated in the dog. Thus, these species may be natural models with which to study the biology of AD, and could also serve as assay systems for Aβ-targeted drugs or new therapeutic strategies against this devastating disease.

Mild cognitive impairment (MCI), and the amnestic subtype of MCI in particular, is the most recent concept used to describe the intermediary state between healthy aging and Alzheimer's disease (AD). It is hoped that research focusing on MCI would yield markers for early identification of individuals with prodromal AD at such a pre-dementia stage when potential disease modifying therapies would be most efficacious. Magnetic resonance imaging (MRI) combined with various data analysis methods provides tools to investigate alterations in brain structure and function in vivo. Structurally, MCI is characterized by atrophy of the medial temporal lobe (MTL) structures such as the hippocampus and entorhinal cortex, and the amount of atrophy in MCI is intermediate between healthy aging and AD. Additionally, atrophy of the posteromedial cortices such as the posterior cingulum and precuneus as well as of the lateral temporal cortices has been reported. The pattern of atrophy appears to vary according to the subtype of MCI. Functional MRI studies in MCI, compared to healthy aging and AD, have demonstrated both increased and decreased MTL activity during encoding novel visually presented material. Differences in the MTL activation pattern in MCI subjects may relate to differences in the severity of cognitive decline. There is some evidence that increased MTL activity observed during encoding may be compensatory due to incipient atrophy in the MTL structures. The resting state (or, “default mode”) network, and the posteromedial cortical regions in particular, appear to malfunction in MCI. It is suggested that both altered MTL and posteromedial cortical function may be indicative of future cognitive decline from MCI to clinical AD.