Beyond the cholinergic hypothesis: do current drugs work in Alzheimer's disease?

In vitro neuroprotective potential of four medicinal plants against rotenone-induced toxicity in SH-SY5Y neuroblastoma cells

BACKGROUND

Lannea schweinfurthii, Zanthoxylum capense, Scadoxus puniceus and Crinum bulbispermum are used traditionally to treat neurological disorders. The aim of this study was to evaluate the cytoprotective potential of the four plants, after induction of toxicity using rotenone, in SH-SY5Y neuroblastoma cells.

METHODS

Cytotoxicity of the plant extracts and rotenone was assessed using the sulforhodamine B (SRB) assay. Fluorometry was used to measure intracellular redox state (reactive oxygen species (ROS) and intracellular glutathione content), mitochondrial membrane potential (MMP) and caspase-3 activity, as a marker of apoptotic cell death.

RESULTS

Of the tested plants, the methanol extract of Z. capense was the least cytotoxic; LC50 121.3 ± 6.97 μg/ml, while S. puniceus methanol extract was the most cytotoxic; LC50 20.75 ± 1.47 μg/ml. Rotenone reduced intracellular ROS levels after 24 h exposure. Pre-treating cells with S. puniceus and C. bulbispermum extracts reversed the effects of rotenone on intracellular ROS levels. Rotenone exposure also decreased intracellular glutathione levels, which was counteracted by pre-treatment with any one of the extracts. MMP was reduced by rotenone, which was neutralized by pre-treatment with C. bulbispermum ethyl acetate extract. All extracts inhibited rotenone-induced activation of caspase-3.

CONCLUSION

The studied plants demonstrated anti-apoptotic activity and restored intracellular glutathione content following rotenone treatment, suggesting that they may possess neuroprotective properties.

Mildronate improves cognition and reduces amyloid-β pathology in transgenic Alzheimer's disease mice

Mildronate, a carnitine congener drug, previously has been shown to provide neuroprotection in an azidothymidine-induced mouse model of neurotoxicity and in a Parkinson's disease rat model. The aim of this study was to investigate the effects of mildronate treatment on cognition and pathology in Alzheimer's disease (AD) model mice (APP(SweDI)). Mildronate was administered i.p. daily at 50 or 100 mg/kg for 28 days. At the end of treatment, the animals were behaviorally and cognitively tested, and brains were assessed for AD-related pathology, inflammation, synaptic markers, and acetylcholinesterase (AChE). The data show that mildronate treatment significantly improved animal performance in water maze and social recognition tests, lowered amyloid-β deposition in the hippocampus, increased expression of the microglia marker Iba-1, and decreased AChE staining, although it did not alter expression of proteins involved in synaptic plasticity (GAP-43, synaptophysin, and GAD67). Taken together, these findings indicate mildronate's ability to improve cognition and reduce amyloid-β pathology in a mouse model of AD and its possible therapeutic utility as a disease-modifying drug in AD patients.

Undifferentiated and differentiated PC12 cells protected by huprines against injury induced by hydrogen peroxide

Oxidative stress is implicated in the pathogenesis of neurodegenerative disorders and hydrogen peroxide (H₂O₂) plays a central role in the stress. Huprines, a group of potent acetylcholinesterase inhibitors (AChEIs), have shown a broad cholinergic pharmacological profile. Recently, it has been observed that huprine X (HX) improves cognition in non transgenic middle aged mice and shows a neuroprotective activity (increased synaptophysin expression) in 3xTg-AD mice. Consequently, in the present experiments the potential neuroprotective effect of huprines (HX, HY, HZ) has been analyzed in two different in vitro conditions: undifferentiated and NGF-differentiated PC12 cells. Cells were subjected to oxidative insult (H₂O₂, 200 µM) and the protective effects of HX, HY and HZ (0.01 µM–1 µM) were analyzed after a pre-incubation period of 24 and 48 hours. All huprines showed protective effects in both undifferentiated and NGF-differentiated cells, however only in differentiated cells the effect was dependent on cholinergic receptors as atropine (muscarinic antagonist, 0.1 µM) and mecamylamine (nicotinic antagonist, 100 µM) reverted the neuroprotection action of huprines. The decrease in SOD activity observed after oxidative insult was overcome in the presence of huprines and this effect was not mediated by muscarinic or nicotinic receptors. In conclusion, huprines displayed neuroprotective properties as previously observed in in vivo studies. In addition, these effects were mediated by cholinergic receptors only in differentiated cells. However, a non-cholinergic mechanism, probably through an increase in SOD activity, seems to be also involved in the neuroprotective effects of huprines.

A new motif in the N-terminal of acetylcholinesterase triggers amyloid-β aggregation and deposition

BACKGROUND AND PURPOSE

As a molecular chaperone, acetylcholinesterase (AChE; EC 3.1.1.7) plays a critical role in the pathogenesis of Alzheimer's disease (AD). The peripheral anionic site (PAS) of AChE has been indicated as the amyloid-β (Aβ) binding domain. The goal of this study was to determine other motifs in AChE involved in Aβ aggregation and deposition.

METHODS AND RESULTS

The β-hairpin in monomeric Aβ is the key motif of nucleation-dependent Aβ self-aggregation. As AChE could induce Aβ aggregation and deposition, we searched AChE for β-hairpin structures. In A11-specific dot blot assay, AChE was detected by an oligomer-specific antibody A11, implying the existence of β-hairpin structures in AChE as β-hairpin was the core motif of oligomers. A molecular superimposing approach further revealed that the N-terminal region, from Glu7 to Ile20, in AChE (AChE 7-20) was similar to the β-hairpin domain in Aβ. The results of further dot blot assays, thioflavin T fluorescence assays, and electron microscopy imaging experiments, indicated that the N-terminal synthetic peptide AChE7-20 had nearly the same ability as AChE with regard to triggering Aβ aggregation and deposition.

CONCLUSIONS

AChE 7-20, a β-hairpin region in AChE, might be a new motif in AChE capable of triggering Aβ aggregation and deposition. This finding will be helpful to design new and more effective Aβ aggregation inhibitors for AD treatment.

Amyloid β, glutamate, excitotoxicity in Alzheimer's disease: are we on the right track?

Alzheimer's disease (AD) has a devastating impact on aged people worldwide. Although sophisticated and advanced molecular methods have been developed for its diagnosis since early phases, pharmacological treatment still represents an unresolved topic. The more the disease progresses, the more the uneffectiveness of antidementia drugs emerges. New and encouraging results from experimental works indicate that glutamate pathway may play a substantial role in the pathogenesis since early stages of the disease. Several experimental data together with the clinical use of the uncompetitive N-methyl-d-aspartate (NMDA) antagonist memantine strengthen this idea. Unfortunately, definitive data on the glutamatergic transmission involvement in AD are still incomplete. Moreover, clinical results indicate only temporarily limited effects of memantine. Currently, memantine is indicated for moderate-to-severe cases of AD, an indication that may limit its efficacy and impact on Alzheimer's dementia. The association of memantine with the acetylcholinesterase inhibitor drugs used to treat dementia symptoms appears to be beneficial, in both experimental and clinical studies. Because cholinergic and glutamatergic dysfunction occurs early in AD, the coadministration of appropriate treatment in early stages of the disease might represent a valid option from the beginning of cognitive decline. Moreover, to better evaluate drug efficacy, the association of the recently introduced biomarkers with a clinical AD profile should be considered an aim to pursue.

Nonoisotopic assay for the presynaptic choline transporter reveals capacity for allosteric modulation of choline uptake

Current therapies to enhance CNS cholinergic function rely primarily on extracellular acetylcholinesterase (AChE) inhibition, a pharmacotherapeutic strategy that produces dose-limiting side effects. The Na(+)-dependent, high-affinity choline transporter (CHT) is an unexplored target for cholinergic medication development. Although functional at the plasma membrane, CHT at steady-state is localized to synaptic vesicles such that vesicular fusion can support a biosynthetic response to neuronal excitation. To identify allosteric potentiators of CHT activity, we mapped endocytic sequences in the C-terminus of human CHT, identifying transporter mutants that exhibit significantly increased transport function. A stable HEK-293 cell line was generated from one of these mutants (CHT LV-AA) and used to establish a high-throughput screen (HTS) compatible assay based on the electrogenic nature of the transporter. We established that the addition of choline to these cells, at concentrations appropriate for high-affinity choline transport at presynaptic terminals, generates a hemicholinium-3 (HC-3)-sensitive, membrane depolarization that can be used for the screening of CHT inhibitors and activators. Using this assay, we discovered that staurosporine increased CHT LV-AA choline uptake activity, an effect mediated by a decrease in choline K(M) with no change in V(max). As staurosporine did not change surface levels of CHT, nor inhibit HC-3 binding, we propose that its action is directly or indirectly allosteric in nature. Surprisingly, staurosporine reduced choline-induced membrane depolarization, suggesting that increased substrate coupling to ion gradients, arising at the expense of nonstoichiometric ion flow, accompanies a shift of CHT to a higher-affinity state. Our findings provide a new approach for the identification of CHT modulators that is compatible with high-throughput screening approaches and presents a novel model by which small molecules can enhance substrate flux through enhanced gradient coupling.

The amyloid precursor protein (APP) family members are key players in S-adenosylmethionine formation by MAT2A and modify BACE1 and PSEN1 gene expression-relevance for Alzheimer's disease

Central hallmark of Alzheimer's disease are senile plaques mainly composed of β-amyloid, which is a cleavage product of the amyloid precursor protein (APP). The physiological function of APP and its family members APLP1 and APLP2 is poorly understood. In order to fill this gap, we established a cell-culture based model with simultaneous knockdown of all members of the family. A comprehensive proteome study of the APP/APLP1/APLP2 knockdown cell lysates versus controls revealed significant protein abundance changes of more than 30 proteins. Targeted validation of selected candidates by immunoblotting supported the significant down-regulation of the methionine adenosyltransferase II, alpha (MAT2A) as well as of peroxiredoxin 4 in the knockdown cells. Moreover, MAT2A was significantly down-regulated at the mRNA level as well. MAT2A catalyzes the production of S-adenosylmethionine from methionine and ATP, which plays a pivotal role in the methylation of neurotransmitters, DNA, proteins, and lipids. MAT2A-dependent significant up-regulation of S-adenosylmethionine was also detectable in the knockdown cells compared with controls. Our results point to a role of the APP family proteins in cellular methylation mechanisms and fit to findings of disturbed S-adenosylmethionine levels in tissue and CSF of Alzheimer disease patients versus controls. Importantly, methylation plays a central role for neurotransmitter generation like acetylcholine pointing to a crucial relevance of our findings for Alzheimer's disease. In addition, we identified differential gene expression of BACE1 and PSEN1 in the knockdown cells, which is possibly a consequence of MAT2A deregulation and may indicate a self regulatory mechanism.

CSF and clinical hallmarks of subcortical dementias: focus on DLB and PDD

Dementia has become a relevant problem associated with the elderly in our countries. Increased interest in the field has yielded a copious literature, so far mostly centered on Alzheimer's dementia. Cerebrospinal fluid (CSF) analysis combined with neuropsychology, even in absence of neuroimaging, represents the gold standard to reach a diagnosis when cortical cognitive impairment prevails. In view of this, low levels of CSF amyloid peptides β (Aβ) and high tau/Aβ protein ratio, despite prominent impairment of executive functions or concomitant vascular burden, facilitate the diagnosis of Alzheimer's disease. Conversely, an early cognitive impairment occurring in patients suffering from Parkinson's disease (PD) or Lewy body disorders (LBDs), both diagnoses posed on pure clinical grounds, remains quite elusive in term of biomarkers or neuropsychological assessment. Whether PD with dementia (PDD) and dementia with Lewy bodies (DLB) represent further steps along with a continuum of the same progressive degeneration due to Lewy bodies deposition, rather then the association of Lewy bodies and Aβ pathology, remains a challenging issue. Aim of this work is to set a state-of-the-art on the neuropsychological profiles of both or DLB. Then, we will focus on the ongoing controversies about the specificity of the standard CSF biomarkers if applied to extrapyramidal disorders. Our conclusions are that the CSF pattern, in PDD and DLB, can certainly be distinct from that in AD, though mechanisms leading to dementia could be shared among them. It is possible that, by combining imaging tracers, neuropsychologically careful assessment and renewed CSF biomarkers, DLB can be better distinguished in subgroups, depending on the presence or absence of a relevant amyloid burden. However, more complete data, possibly collected in fieri during the progressive derangement of cognitive abilities, are needed to improve our ability to decipher and treat these entities.

Long-term methionine exposure induces memory impairment on inhibitory avoidance task and alters acetylcholinesterase activity and expression in zebrafish (Danio rerio)

Hypermethioninemic patients exhibit a variable degree of neurological dysfunction. However, the mechanisms involved in these alterations have not been completely clarified. Cholinergic system has been implicated in many physiological processes, including cognitive performances, as learning, and memory. Parameters of cholinergic signaling have already been characterized in zebrafish brain. Since zebrafish is a small freshwater teleost which is a vertebrate model for modeling behavioral and functional parameters related to human pathogenesis and for clinical treatment screenings, in the present study we investigated the effects of short- and long-term methionine exposure on cognitive impairment, AChE activity and gene expression in zebrafish. For the studies, animals were exposed at two methionine concentrations (1.5 and 3.0 mM) during 1 h or 7 days (short- or long-term treatments, respectively). We observed a significant increase in AChE activity of zebrafish brain membranes after long-term methionine exposure at 3.0 mM. However, AChE gene expression decreased significantly in both concentrations tested after 7 days of treatment, suggesting that post-translational events are involved in the enhancement of AChE activity. Methionine treatment induces memory deficit in zebrafish after long-term exposure to this amino acid, which could be related, at least in part, with cognitive impairment observed in hypermethioninemia. Therefore, the results here presented raise a new perspective to use the zebrafish as a complementary vertebrate model for studying inborn errors of metabolism, which may help to better understand the pathophysiology of this disease.

Role of serotonin in Alzheimer's disease: a new therapeutic target?

Mounting evidence accumulated over the past few years indicates that the neurotransmitter serotonin plays a significant role in cognition. As a drug target, serotonin receptors have received notable attention due in particular to the role of several serotonin-receptor subclasses in cognition and memory. The intimate anatomical and neurochemical association of the serotonergic system with brain areas that regulate memory and learning has directed current drug discovery programmes to focus on this system as a major therapeutic drug target. Thus far, none of these programmes has yielded unambiguous data that suggest that any of the new drug entities possesses disease-modifying properties, and significantly more research in this promising area of investigation is required. Compounds are currently being investigated for activity against serotonin 5-HT(1), 5-HT(4) and 5-HT(6) receptors. This review concludes that most work done in the development of selective serotonin receptor ligands is in the pre-clinical or early clinical phase. Also, while many of these compounds will likely find application as adjuvant therapy in the symptomatic treatment of Alzheimer's disease, there are currently only a few drug entities with activity against serotonin receptors that may offer the potential to alter the progression of the disease.