Beta-secretase BACE1 is differentially controlled through muscarinic acetylcholine receptor signaling

α7 nicotinic acetylcholine receptors are necessary for basal forebrain activation to increase expression of the nerve growth factor receptor TrkA

Activation of the basal forebrain leads to increases in the expression of the nerve growth factor receptor, Tropomyosin receptor kinase A (TrkA) and decreases in expression of the beta amyloid cleavage enzyme 1 (BACE1) in the cerebral cortex of both sexes of 5xFAD mice. The studies described in this report were designed to determine if these changes were dependent on acetylcholine receptors. Mice were stimulated unilaterally in the basal forebrain for two weeks. Animals were administered a cholinergic antagonist, or saline, 30 minutes prior to stimulation. Animals administered saline exhibited significant increases in TrkA expression and decreases in BACE1 in the stimulated hemisphere relative to the unstimulated. While both nonselective nicotinic and muscarinic acetylcholine receptor blockade attenuated the BACE1 decline, only the nicotinic receptor antagonism blocked the TrkA increase. Next, we applied selective nicotinic antagonists, and the α7 antagonist blocked the TrkA increases, but the α4β2 antagonist did not. BACE1 declines were not blocked by either intervention. Mice with a loxP conditional knockout of the gene for the α7 nicotinic receptor were also employed in these studies. Animals were either stimulated bilaterally for two weeks, or left unstimulated. With or without stimulation, the expression of TrkA receptors was lower in the cortical region with the α7 nicotinic receptor knockdown. We thus conclude that α7 nicotinic receptor activation is necessary for normal expression of TrkA and increases caused by basal forebrain activation, while BACE1 declines caused by stimulation have dependency on a broader array of receptor subtypes.

Cholinergic Receptor Modulation as a Target for Preventing Dementia in Parkinson's Disease

Parkinson’s disease (PD) is a neurodegenerative condition characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) in the midbrain resulting in progressive impairment in cognitive and motor abilities. The physiological and molecular mechanisms triggering dopaminergic neuronal loss are not entirely defined. PD occurrence is associated with various genetic and environmental factors causing inflammation and mitochondrial dysfunction in the brain, leading to oxidative stress, proteinopathy, and reduced viability of dopaminergic neurons. Oxidative stress affects the conformation and function of ions, proteins, and lipids, provoking mitochondrial DNA (mtDNA) mutation and dysfunction. The disruption of protein homeostasis induces the aggregation of alpha-synuclein (α-SYN) and parkin and a deficit in proteasome degradation. Also, oxidative stress affects dopamine release by activating ATP-sensitive potassium channels. The cholinergic system is essential in modulating the striatal cells regulating cognitive and motor functions. Several muscarinic acetylcholine receptors (mAChR) and nicotinic acetylcholine receptors (nAChRs) are expressed in the striatum. The nAChRs signaling reduces neuroinflammation and facilitates neuronal survival, neurotransmitter release, and synaptic plasticity. Since there is a deficit in the nAChRs in PD, inhibiting nAChRs loss in the striatum may help prevent dopaminergic neurons loss in the striatum and its pathological consequences. The nAChRs can also stimulate other brain cells supporting cognitive and motor functions. This review discusses the cholinergic system as a therapeutic target of cotinine to prevent cognitive symptoms and transition to dementia in PD.

Neuropharmacology of Cevimeline and Muscarinic Drugs-Focus on Cognition and Neurodegeneration

At present, Alzheimer’s disease (AD) and related dementias cannot be cured. Therefore, scientists all over the world are trying to find a new approach to prolong an active life of patients with initial dementia. Both pharmacological and non-pharmacological pathways are investigated to improve the key symptom of the disease, memory loss. In this respect, influencing the neuromodulator acetylcholine via muscarinic receptors, such as cevimeline, might be one of the therapeutic alternatives. The purpose of this study is to explore the potential of cevimeline on the cognitive functions of AD patients. The methodology is based on a systematic literature review of available studies found in Web of Science, PubMed, Springer, and Scopus on the research topic. The findings indicate that cevimeline has shown an improvement in experimentally induced cognitive deficits in animal models. Furthermore, it has demonstrated to positively influence tau pathology and reduce the levels of amyloid-β (Aβ) peptide in the cerebral spinal fluid of Alzheimer’s patients. Although this drug has not been approved by the FDA for its use among AD patients and there is a lack of clinical studies confirming and extending this finding, cevimeline might represent a breakthrough in the treatment of AD.

Alterations and interactions of subcortical modulatory systems in Alzheimer's disease

The pathogenesis of Alzheimer's disease (AD) is not fully understood. Here we summarize current knowledge on the involvement of the serotonergic, noradrenergic, dopaminergic, cholinergic, and opioid systems in AD, emphasizing the importance of interactions between the serotonergic and the other subcortical modulatory systems during the progression of AD. In physiological conditions, all neurotransmitter systems function in concert and are interdependent at both the neuroanatomical and molecular levels. Through their early involvement in AD, cognitive and behavioral abilities that rely on their interactions also become disrupted. Considering that serotonin (5HT) regulates the release of noradrenaline (NA), dopamine (DA) and acetylcholine (ACh), any alteration in 5HT levels leads to disturbance of NA, DA, and ACh homeostasis in the brain. One of the earliest pathological changes during the prodromal phase of AD is a decrease of serotonergic transmission throughout the brain, with serotonergic receptors being also affected. Additionally, serotonergic and noradrenergic as well as serotonergic and dopaminergic nuclei are reciprocally interconnected. As the serotonergic dorsal raphe nucleus (DRN) is affected by pathological changes early in AD, and the noradrenergic locus coeruleus (LC) and dopaminergic ventral tegmental area (VTA) exhibit AD-related pathological changes, their connectivity also becomes altered in AD. Such disrupted interactions among neurotransmitter systems in AD can be used in the development of multi-target drugs. Some of the potential AD therapeutics (such as ASS234, RS67333, tropisetron) target multiple neurotransmitter systems to achieve the best possible improvement of cognitive and behavioral deficits observed in AD. Here, we review how serotonergic system interacts with other subcortical modulatory systems (noradrenergic, dopaminergic, cholinergic, and opioid systems) during AD.

Amyloid-β, tau, and the cholinergic system in Alzheimer's disease: seeking direction in a tangle of clues

The link between histopathological hallmarks of Alzheimer's disease (AD), i.e. amyloid plaques, and neurofibrillary tangles, and AD-associated cognitive impairment, has long been established. However, the introduction of interactions between amyloid-beta (Aβ) as well as hyperphosphorylated tau, and the cholinergic system to the territory of descriptive neuropathology has drastically changed this field by adding the theory of synaptic neurotransmission to the toxic pas de deux in AD. Accumulating data show that a multitarget approach involving all amyloid, tau, and cholinergic hypotheses could better explain the evolution of events happening in AD. Various species of both Aβ and tau could be traced in cholinergic neurons of the basal forebrain system early in the course of the disease. These molecules induce degeneration in the neurons of this system. Reciprocally, aberrant cholinergic system modulation promotes changes in amyloid precursor protein (APP) metabolism and tau phosphorylation, resulting in neurotoxicity, neuroinflammation, and neuronal death. Altogether, these changes may better correlate with the clinical findings and cognitive impairment detected in AD patients. Failure of several of Aβ- and tau-related therapies further highlights the need for special attention to molecules that target all of these mentioned pathologic changes. Another noteworthy fact here is that none of the popular hypotheses of AD such as amyloidopathy or tauopathy seem to be responsible for the changes observed in AD alone. Thus, the main culprit should be sought higher in the stream somewhere in APP metabolism or Wnt signaling in the cholinergic system of the basal forebrain. Future studies should target these pathological events.

Role of GPCR signaling and calcium dysregulation in Alzheimer's disease

Alzheimer's disease (AD), a late onset neurodegenerative disorder is characterized by the loss of memory, disordered cognitive function, caused by accumulation of amyloid-β (Aβ) peptide and neurofibrillary tangles (NFTs) in the neocortex and hippocampal brain area. Extensive research has been done on the findings of the disease etiology or pathological causes of aggregation of Aβ and hyperphosphorylation of tau protein without much promising results. Recently, calcium dysregulation has been reported to play an important role in the pathophysiology of AD. Calcium ion acts as one of the major secondary messengers, regulates many signaling pathways involved in cell survival, proliferation, differentiation, transcription and apoptosis. Calcium signaling is one of the major signaling pathways involved in the formation of memory, generation of energy and other physiological functions. It also can modulate function of many proteins upon binding. Dysregulation in calcium homeostasis leads to many physiological changes leading to neurodegenerative diseases including AD. In AD, GPCRs generate secondary messengers which regulate calcium homeostasis inside the cell and is reported to be disturbed in the pathological condition. Calcium channels and receptors present on the plasma membrane and intracellular organelle maintain calcium homeostasis through different signaling mechanisms. In this review, we have summarized the different calcium channels and receptors involved in calcium dysregulation which in turn play a critical role in the pathogenesis of AD. Understanding the role of calcium channels and GPCRs to maintain calcium homeostasis is an attempt to develop effective AD treatments.

Constipation in Tg2576 mice model for Alzheimer's disease associated with dysregulation of mechanism involving the mAChR signaling pathway and ER stress response

BACKGROUND

Although constipation has been researched in various neurological disorders, including Parkinson's disease (PD) and spinal cord injury (SCI), the pathological mechanism of this symptom has not been investigated in Alzheimer's disease (AD) associated with loss of nerve cells in the brain. This study was undertaken to gain scientific evidences for a molecular correlation between constipation and AD.

METHODS

To understand the etiology, we measured alterations in various constipation parameters, muscarinic acetylcholine receptors (mAChRs) and endoplasmic reticulum (ER) stress response, in 11-month-old Tg2576 transgenic (Tg) mice showing AD-like phenotypes.

RESULTS

A high accumulation of amyloid beta (Aβ) peptides, a key marker of AD pathology, were detected in the cortex and hippocampus of Tg mice. Furthermore, significant alterations were observed in various constipation parameters including stool weight, histological structure, cytological structure and mucin secretion in Tg2576 mice. Moreover, M2 and M3 expression and the downstream signaling pathways of mAChRs were decreased in the Tg group, as compared with non-Tg (NT) group. Furthermore, activation of ER stress proteins and alteration of ER structure were also detected in the same group.

CONCLUSIONS

The results of the present study provide strong novel evidence that the neuropathological constipation detected in Tg2576 mice is linked to dysregulation of the mAChR signaling pathways and ER stress response.

Chalcone-based carbamates for Alzheimer's disease treatment

AIM

Alzheimer's disease is a still untreatable multifaceted pathology, and drugs able to stop or reverse its progression are urgently needed. In this picture, the recent reformulation of the cholinergic hypothesis renewed the interest for acetylcholinesterase inhibitors. In this paper, a series of naturally inspired chalcone-based carbamates was designed to target cholinesterase enzymes and possibly generate fragments endowed with neuroprotective activity in situ. Results & methodology: All compounds presented in this study showed nanomolar potency for cholinesterase inhibition. Notably, fragment 11d also displayed an interesting neuroprotective profile.

CONCLUSION

These new derivatives are able to simultaneously modulate different key targets involved in Alzheimer's disease, and could be regarded as promising starting points for the development of disease-modifying drug candidates. [Formula: see text].

Alzheimer's disease: Targeting the Cholinergic System

Acetylcholine (ACh) has a crucial role in the peripheral and central nervous systems. The enzyme choline acetyltransferase (ChAT) is responsible for synthesizing ACh from acetyl-CoA and choline in the cytoplasm and the vesicular acetylcholine transporter (VAChT) uptakes the neurotransmitter into synaptic vesicles. Following depolarization, ACh undergoes exocytosis reaching the synaptic cleft, where it can bind its receptors, including muscarinic and nicotinic receptors. ACh present at the synaptic cleft is promptly hydrolyzed by the enzyme acetylcholinesterase (AChE), forming acetate and choline, which is recycled into the presynaptic nerve terminal by the high-affinity choline transporter (CHT1). Cholinergic neurons located in the basal forebrain, including the neurons that form the nucleus basalis of Meynert, are severely lost in Alzheimer’s disease (AD). AD is the most ordinary cause of dementia affecting 25 million people worldwide. The hallmarks of the disease are the accumulation of neurofibrillary tangles and amyloid plaques. However, there is no real correlation between levels of cortical plaques and AD-related cognitive impairment. Nevertheless, synaptic loss is the principal correlate of disease progression and loss of cholinergic neurons contributes to memory and attention deficits. Thus, drugs that act on the cholinergic system represent a promising option to treat AD patients.

BACE-1 is expressed in the blood-brain barrier endothelium and is upregulated in a murine model of Alzheimer's disease

Endothelial cells of the blood-brain barrier form a structural and functional barrier maintaining brain homeostasis via paracellular tight junctions and specific transporters such as P-glycoprotein. The blood-brain barrier is responsible for negligible bioavailability of many neuroprotective drugs. In Alzheimer's disease, current treatment approaches include inhibitors of BACE-1 (β-site of amyloid precursor protein cleaving enzyme), a proteinase generating neurotoxic β-amyloid. It is known that BACE-1 is highly expressed in endosomes and membranes of neurons and glia. We now provide evidence that BACE-1 is expressed in blood-brain barrier endothelial cells of human, mouse, and bovine origin. We further show its predominant membrane localization by 3D-dSTORM super-resolution microscopy, and by biochemical fractionation that further shows an abluminal distribution of BACE-1 in brain microvessels. We confirm its functionality in processing APP in primary mouse brain endothelial cells. In an Alzheimer's disease mouse model we show that BACE-1 is upregulated at the blood-brain barrier compared to healthy controls. We therefore suggest a critical role for BACE-1 at the blood-brain barrier in β-amyloid generation and in vascular aspects of Alzheimer's disease, particularly in the development of cerebral amyloid angiopathy.

Cholinergic Mechanisms in the Cerebral Cortex: Beyond Synaptic Transmission

Functional overviews of cholinergic mechanisms in the cerebral cortex have traditionally focused on the release of acetylcholine with modulator and transmitter effects. Recently, however, data have emerged that extend the role of acetylcholine and cholinergic innervations to a range of housekeeping and metabolic functions. These include regulation of amyloid precursor protein (APP) processing with production of amyloid β (Aβ) and other APP fragments and control of the phosphorylation of microtubule-associated protein (MAP) tau. Evidence has been also presented for receptor-ligand like interactions of cholinergic receptors with soluble Aβ peptide and MAP tau, with modulator and signaling effects. Moreover, high-affinity binding of Aβ to the neurotrophin receptor p75 (p75NTR) enriched in basalo-cortical cholinergic projections has been implicated in clearance of Aβ and nucleation of amyloid plaques. Here, we critically evaluate these unorthodox cholinergic mechanisms and discuss their role in neuronal physiology and the biology of Alzheimer's disease.

G Protein-Coupled Receptors (GPCRs) in Alzheimer's Disease: A Focus on BACE1 Related GPCRs

The G protein coupled receptors (GPCRs) have been considered as one of the largest families of validated drug targets, which involve in almost overall physiological functions and pathological processes. Meanwhile, Alzheimer’s disease (AD), the most common type of dementia, affects thinking, learning, memory and behavior of elderly people, that has become the hotspot nowadays for its increasing risks and incurability. The above fields have been intensively studied, and the link between the two has been demonstrated, whereas the way how GPCRs perturb AD progress are yet to be further explored given their complexities. In this review, we summarized recent progress regarding the GPCRs interacted with β-site APP cleaving enzyme 1 (BACE1), a key secretase in AD pathogenesis. Then we discussed the current findings on the regulatory roles of GPCRs on BACE1, and the possibility for pharmaceutical treatment of AD patients by the allosteric modulators and biased ligands of GPCRs. We hope this review can provide new insights into the understanding of mechanistic link between GPCRs and BACE1, and highlight the potential of GPCRs as therapeutic target for AD.

M1 muscarinic acetylcholine receptor in Alzheimer's disease

The degeneration of cholinergic neurons and cholinergic hypofunction are pathologies associated with Alzheimer's disease (AD). Muscarinic acetylcholine receptors (mAChRs) mediate acetylcholine-induced neurotransmission and five mAChR subtypes (M1-M5) have been identified. Among them, M1 mAChR is widely expressed in the central nervous system and has been implicated in many physiological and pathological brain functions. In addition, M1 mAChR is postulated to be an important therapeutic target for AD and several other neurodegenerative diseases. In this article, we review recent progress in understanding the functional involvement of M1 mAChR in AD pathology and in developing M1 mAChR agonists for AD treatment.

Targeting selective activation of M(1) for the treatment of Alzheimer's disease: further chemical optimization and pharmacological characterization of the M(1) positive allosteric modulator ML169

The M(1) muscarinic acetylcholine receptor is thought to play an important role in memory and cognition, making it a potential target for the treatment of Alzheimer's disease (AD) and schizophrenia. Moreover, M(1) interacts with BACE1 and regulates its proteosomal degradation, suggesting selective M(1) activation could afford both palliative cognitive benefit as well as disease modification in AD. A key challenge in targeting the muscarinic acetylcholine receptors is achieving mAChR subtype selectivity. Our lab has previously reported the M(1) selective positive allosteric modulator ML169. Herein we describe our efforts to further optimize this lead compound by preparing analogue libraries and probing novel scaffolds. We were able to identify several analogues that possessed submicromolar potency, with our best example displaying an EC(50) of 310 nM. The new compounds maintained complete selectivity for the M(1) receptor over the other subtypes (M(2)-M(5)), displayed improved DMPK profiles, and potentiated the carbachol (CCh)-induced excitation in striatal MSNs. Selected analogues were able to potentiate CCh-mediated nonamyloidogenic APPsα release, further strengthening the concept that M(1) PAMs may afford a disease-modifying role in the treatment of AD.

Relationships between the amyloid precursor protein and its various proteolytic fragments and neuronal systems

Alzheimer's disease (AD) is a progressive neurodegenerative disease and in its familial form is associated with mutations in the amyloid precursor protein (APP) and the presenilins (PSs). Much data regarding the interactions of APP, its proteolytic fragments and PS have been generated, expanding our understanding of the roles of these proteins in mechanisms underlying cognitive function and revealing many complex relationships with wide ranging cellular systems. In this review, we examine the multiple interactions of APP and its proteolytic fragments with other neuronal systems in terms of feedback loops and use these relationships to build a map. We highlight the complexity involved in the APP proteolytic system and discuss alternative perspectives on the roles of APP and its proteolytic fragments in dynamic processes associated with disease progression in AD. We highlight areas where data are missing and suggest potential confounding factors. We suggest that a systems biology approach enhances representations of the data and may be more useful in modelling both normal cognition and disease processes.

M1 muscarinic acetylcholine receptor interacts with BACE1 and regulates its proteosomal degradation

A prime culprit in the pathogenesis of Alzheimer's disease (AD) is overproduction/aggregation of β-amyloid (Aβ), which is derived from β-Amyloid Precursor Protein through sequential cleavages by β-site APP cleaving protein 1 (BACE1) and γ-secretase. The level/activity of BACE1 is elevated in sporadic AD and identification of proteins that affect BACE1 is important in AD research. Here we found that M1 Muscarinic acetylcholine receptor (M1 mAChR), an important G protein-coupled receptor involved in cholinergic neuronal activity, can interact with BACE1 and mediate its proteosomal degradation. Moreover, overexpression and downregulation of M1 mAChR can decrease and increase the levels of BACE1, as well as the generation of Aβ, respectively. These findings point to a novel coupling of BACE1 and M1 mAChR in AD and possibly schizophrenia.

The Basic Biology of BACE1: A Key Therapeutic Target for Alzheimer's Disease

Alzheimer’s disease (AD) is an intractable, neurodegenerative disease that appears to be brought about by both genetic and non-genetic factors. The neuropathology associated with AD is complex, although amyloid plaques composed of the β-amyloid peptide (Aβ) are hallmark neuropathological lesions of AD brain. Indeed, Aβ plays an early and central role in this disease. β-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is the initiating enzyme in Aβ genesis and BACE1 levels are elevated under a variety of conditions. Given the strong correlation between Aβ and AD, and the elevation of BACE1 in this disease, this enzyme is a prime drug target for inhibiting Aβ production in AD. However, nine years on from the initial identification of BACE1, and despite intense research, a number of key questions regarding BACE1 remain unanswered. Indeed, drug discovery and development for AD continues to be challenging. While current AD therapies temporarily slow cognitive decline, treatments that address the underlying pathologic mechanisms of AD are completely lacking. Here we review the basic biology of BACE1. We pay special attention to recent research that has provided some answers to questions such as those involving the identification of novel BACE1 substrates, the potential causes of BACE1 elevation and the putative function of BACE1 in health and disease. Our increasing understanding of BACE1 biology should aid the development of compounds that interfere with BACE1 expression and activity and may lead to the generation of novel therapeutics for AD.

Decreased Neuregulin 1 C-terminal fragment in Brodmann's area 6 of patients with schizophrenia

Neuregulin 1 (NRG1) is a susceptibility gene for schizophrenia. A decrease in NRG1-ErbB4 signalling has also been associated with the disease. β-amyloid precursor protein-cleaving enzyme (BACE1) processes type III NRG1 precursor, a major neuregulin variant expressed in the brain, to release NRG1 fragments that trigger signalling events and activation of neurotransmitter receptors. Experimental evidence suggests that muscarinic acetylcholine receptors (CHRM) regulate BACE1 expression. Having recently shown that CHRM1 levels are decreased selectively in frontal cortex regions of a subpopulation of schizophrenic patients (muscarinic receptor deficit schizophrenia, MRDS) we aimed to compare the protein expression of BACE1 and NRG1 in the agranular frontal cortex Brodmann's area 6 of SCZ subjects with normal levels of CHRM1 (N = 19), MRDS (N = 20), and age/gender-matched non-psychiatric (healthy) controls (HC; N = 20). Western blot analysis of post-mortem samples showed that the levels of BACE1 and full-length NRG1 precursor (130 kDa) did not differ significantly between the three groups. In contrast, the levels of the NRG1 C-terminal fragment (NRG1-CTF) were decreased by approximately 50% in both schizophrenic groups compared to the HC group (p<0.0027). The ratio of NRG1-CTF versus NRG1 precursor was significantly reduced in the SCZ groups compared to the HC group (p = 0.051). There was no correlation between the levels of either full-length NRG1, NRG1-CTF, or BACE1 and the final recorded doses of antipsychotic drugs for the subjects with schizophrenia. A positive correlation was found between BACE1 and full-length NRG1 precursor in the HC group (r(2) = 0.671, p<0.001) but not in the schizophrenic groups. These data suggest that the proteolytic processing of NRG1 is impaired in schizophrenia.

The hAPP-YAC transgenic model has elevated UPS activity in the frontal cortex similar to Alzheimer's disease and Down's syndrome

The ubiquitin-proteasome system (UPS) is critical for handling the intra-cellular load of abnormal and misfolded proteins in several neurodegenerative diseases. First, to determine the effects of the over-expression of human amyloid precursor protein (hAPP) on UPS, we measured proteasome activities using fluorescent substrates in the frontal cortex of hAPP-yeast artificial chromosome (YAC) transgenic (tg) mice (R1.40, hemizygous; Lamb, Nat Genet, 9, 4; 1995). Chymotrypsin and PGPH-like activities of proteasome were increased in frontal cortex of hAPP-YAC tg mice. These proteasome activities (both chymotrypsin and PGPH-like) were further increased by cholinergic stimulation in littermate control mice, but not in hAPP-YAC tg mice. Nerve growth factor (NGF) levels were decreased by hAPP over-expression in the frontal cortex and hippocampus of hAPP-YAC tg mice, and further decreased by M1 agonist treatment in the hippocampus of littermate control and hAPP-YAC tg mice. Interestingly, the frontal cortex (BA9 area) of Alzheimer's disease (AD) patients (Stage 3, n=11) and Down's syndrome (DS) patients (n=9) showed similar up-regulation of the UPS activities to those seen in hAPP-YAC tg mice. M1 agonist stimulation increased the activities of α-secretase, which were down-regulated by hAPP over-expression in the frontal cortex of hAPP-YAC tg mice. These results demonstrate that (i) hAPP-YAC tg mice have an up-regulation in the frontal cortex of the UPS similar to AD and DS patients; (ii) muscarinic stimulation increase UPS activities, increase secreted APP (APPs) levels, and decrease amyloid beta 42/40 ratio only in littermate controls, but not in hAPP-YAC tg mice. Taken together, these results suggest that both the adaptive reactions in the proteostatic network and pathological changes in AD and DS need to be considered in the future potential therapeutics.

Time for a change in the research paradigm for Alzheimer's disease: the value of a chaotic matrix modeling approach

The amyloid cascade hypothesis, based on the genetic data from early onset, familial forms of the disease, has been the dominant model for many years and involves over production and deposition of the beta amyloid protein as causal in the disease process. However, it does not apply very well to the more common, later onset, sporadic form of the disease, where a wider range of factors appear to be involved in disease progression. Over recent years, data illustrating reciprocal interactions between the amyloid precursor protein (APP) and its various metabolites with many factors involved in normal synaptic plasticity have emerged. These feedback relationships have the potential to affect the complex kinase cascades involved in every aspect of neuronal function. Further, data regarding the multiple roles of the presenilins have the potential to allow the over expression and deposition of the amyloid beta protein to be both a cause and consequence of disease progression, with relevance in both sporadic and familial of Alzheimer's disease (AD). Disease progression might be better explained by a chaotic matrix of factors and raises the question again whether AD should be approached as a single entity or as a syndrome, with important consequences for disease identification and treatment.

Beta-secretase: structure, function, and evolution

The most popular current hypothesis is that Alzheimer's disease (AD) is caused by aggregates of the amyloid peptide (Abeta), which is generated by cleavage of the Abeta protein precursor (APP) by beta-secretase (BACE-1) followed by gamma-secretase. BACE-1 cleavage is limiting for the production of Abeta, making it a particularly good drug target for the generation of inhibitors that lower Abeta. A landmark discovery in AD was the identification of BACE-1 (a.k.a. Memapsin-2) as a novel class of type I transmembrane aspartic protease. Although BACE-2, a homologue of BACE-1, was quickly identified, follow up studies using knockout mice demonstrated that BACE-1 was necessary and sufficient for most neuronal Abeta generation. Despite the importance of BACE-1 as a drug target, development has been slow due to the incomplete understanding of its function and regulation and the difficulties in developing a brain penetrant drug that can specifically block its large catalytic pocket. This review summarizes the biological properties of BACE-1 and attempts to use phylogenetic perspectives to understand its function. The article also addresses the challenges in discovering a selective drug-like molecule targeting novel mechanisms of BACE-1 regulation.

Cholinergic and glutamatergic alterations beginning at the early stages of Alzheimer disease: participation of the phospholipase A2 enzyme

RATIONALE

Alzheimer disease (AD), a progressive neurodegenerative disorder, is the leading cause of dementia in the elderly. A combination of cholinergic and glutamatergic dysfunction appears to underlie the symptomatology of AD, and thus, treatment strategies should address impairments in both systems. Evidence suggests the involvement of phospholipase A(2) (PLA(2)) enzyme in memory impairment and neurodegeneration in AD via actions on both cholinergic and glutamatergic systems.

OBJECTIVES

To review cholinergic and glutamatergic alterations underlying cognitive impairment and neuropathology in AD and attempt to link PLA(2) with such alterations.

METHODS

Medline databases were searched (no date restrictions) for published articles with links among the terms Alzheimer disease (mild, moderate, severe), mild cognitive impairment, choline acetyltransferase, acetylcholinesterase, NGF, NGF receptor, muscarinic receptor, nicotinic receptor, NMDA, AMPA, metabotropic glutamate receptor, atrophy, glucose metabolism, phospholipid metabolism, sphingolipid, membrane fluidity, phospholipase A(2), arachidonic acid, attention, memory, long-term potentiation, beta-amyloid, tau, inflammation, and reactive species. Reference lists of the identified articles were checked to identify additional studies of interest.

RESULTS

Overall, results suggest the hypothesis that persistent inhibition of cPLA(2) and iPLA(2) isoforms at early stages of AD may play a central role in memory deficits and beta-amyloid production through down-regulation of cholinergic and glutamate receptors. As the disease progresses, beta-amyloid induced up-regulation of cPLA(2) and sPLA(2) isoforms may play critical roles in inflammation and oxidative stress, thus participating in the neurodegenerative process.

CONCLUSION

Activation and inhibition of specific PLA(2) isoforms at different stages of AD could be of therapeutic importance and delay cognitive dysfunction and neurodegeneration.

Neurosecretases provide strategies to treat sporadic and familial Alzheimer disorders

Recent discoveries on neurosecretases and their trafficking to release fibril-forming neuropeptides or other products, are of interest to pathology, cell signaling and drug discovery. Nomenclature arose from the use of amyloid precursor protein (APP) as a prototypic type-1 substrate leading to the isolation of beta-secretase (BACE), multimeric complexes (gamma-secretase, gamma-SC) for intramembranal cleavage, and attributing a new function to well-characterized metalloproteases of the ADAM family (alpha-secretase) for normal APP turnover. While purified alpha/beta-secretases facilitate drug discovery, gamma-SC presents greater challenges for characterization and mechanisms of catalysis. The review comments on links between mutation or polymorphisms in relation to enzyme mechanisms and disease. The association between lipoprotein receptor LRP11 variants and sporadic Alzheimer's disease (SAD) offers scope to integrate components of pre- and post-Golgi membranes, or brain clathrin-coated vesicles within pathways for trafficking as targets for intervention. The presence of APP and metabolites in brain clathrin-coated vesicles as significant cargo with lipoproteins and adaptors focuses attention as targets for therapeutic intervention. This overview emphasizes the importance to develop new therapies targeting neurosecretases to treat a major neurological disorder that has vast economic and social implications.

The significance of the cholinergic system in the brain during aging and in Alzheimer's disease

Acetylcholine is widely distributed in the nervous system and has been implicated to play a critical role in cerebral cortical development, cortical activity, controlling cerebral blood flow and sleep-wake cycle as well as in modulating cognitive performances and learning and memory processes. Cholinergic neurons of the basal forebrain complex have been described to undergo moderate degenerative changes during aging, resulting in cholinergic hypofunction that has been related to the progressing memory deficits with aging. Basal forebrain cholinergic cell loss is also a consistent feature of Alzheimer's disease, which has been suggested to cause, at least partly, the cognitive deficits observed, and has led to the formulation of the cholinergic hypotheses of geriatric memory dysfunction. Impaired cortical cholinergic neurotransmission may also contribute to beta-amyloid plaque pathology and increase phosphorylation of tau protein the main component of neurofibrillar tangles in Alzheimer's disease. Understanding the molecular mechanisms underlying the interrelationship between cortical cholinergic dysfunction, beta-amyloid formation and deposition, and tau pathology in Alzheimer's disease, would allow to derive potential treatment strategies to pharmacologically intervene in the disease-causing signaling cascade.

Transcriptional and translational regulation of BACE1 expression--implications for Alzheimer's disease

The proteolytical processing of the amyloid precursor protein (APP) gives rise to beta-amyloid peptides, which accumulate in brains of Alzheimer's disease (AD) patients. Different soluble or insoluble higher molecular weight forms of beta-amyloid peptides have been postulated to trigger a complex pathological cascade that may cause synaptic dysfunction, inflammatory processes, neuronal loss, cognitive impairment, and finally the onset of the disease. The generation of beta-amyloid peptides requires the proteolytical cleavage of APP by an aspartyl protease named beta-site APP-cleaving enzyme 1 (BACE1). The expression and enzymatic activity of BACE1 are increased in brains of AD patients. Here we discuss the importance of a number of recently identified transcription factors as well as post-transcriptional modifications and activation of intracellular signaling molecules for the regulation of BACE1 expression in brain. Importantly, some of these factors are known to be involved in the inflammatory and chronic stress responses of the brain, which are compromised during aging. Moreover, recent evidence indicates that beneficial effects of non-steriodal anti-inflammatory drugs on the progression of AD are mediated--at least in part--by effects on the peroxisome proliferator-activated receptor-gamma response element present in the BACE1 promoter. The identification of the cell type-specific expression and activation of NF-kappaB, Sp1 and YY1 transcription factors may provide a basis to specifically interfere with BACE1 expression and, thereby, to lower the concentrations of beta-amyloid peptides, which may prevent neuronal cell loss and cognitive decline in AD patients.

Cholinergic and glutamatergic drugs in Alzheimer's disease therapy

The pathology and clinical symptoms of Alzheimer's disease are well known and include plaques, tangles, cell loss and dysfunction. The target of current treatments is to improve neuronal dysfunction and produce symptomatic benefits based on a clear understanding of neurotransmitter biochemistry. The purpose of this review is to examine the scientific background to currently available treatments, discuss the clinical experience of employing these drugs in Alzheimer's disease patients and review the socioeconomic influences on their use in the future.

Zinc uptake is mediated by M1 muscarinic acetylcholine receptors in differentiated SK-SH-SY5Y cells

Alzheimer's disease (AD), the most prevalent form of dementia, is characterized by several major morphological hallmarks such as senile plaques, neurofibrillary tangles and a loss of cholinergic basal forebrain neurons. Apart from cholinergic markers like choline acetyltransferase and acetylcholinesterase, there have been reports on changes in muscarinic acetylcholine receptors (mAChR) as well as on influences of zinc metabolism in the disease. As recent studies gave hints about a possible link between mAChRs and zinc uptake, the human neuroblastoma cell line SK-SH-SY5Y was used to evaluate the role of M1-mAChR on zinc uptake. Zinc levels were semi-quantitatively detected by using the zinc-specific fluorophor Zn-AF2-DA. In the presence of 1 microM extracellular zinc, M1-mAChR stimulation with talsaclidine increased intracellular zinc levels as did stimulation of PKC by phorbol esters. Furthermore, the effect of extracellular zinc on the expression of the zinc finger protein PNUTS (protein phosphatase 1 nuclear targeting subunit 10) was investigated and revealed an upregulation of PNUTS expression in the presence of 1 microM extracellular zinc by 294% when compared to incubation in zinc free medium. In summary, this report demonstrates that intracellular zinc uptake in SK-SH-SY5Y cells is controlled by M1-mAChR mediated signalling pathways and that zinc may act as a cofactor for transcriptional regulation of zinc finger genes such as PNUTS.

Basal forebrain cholinergic dysfunction in Alzheimer's disease--interrelationship with beta-amyloid, inflammation and neurotrophin signaling

Alzheimer's disease, the most common neurodegenerative disorder of senile dementia, is characterized by two major morpho-pathological hallmarks. Deposition of extracellular neuritic, beta-amyloid peptide-containing plaques (senile plaques) in cerebral cortical regions of Alzheimer patients is accompanied by the presence of intracellular neurofibrillary tangles in cerebral pyramidal neurons. Basal forebrain cholinergic dysfunction is also a consistent feature of Alzheimer's disease, which has been suggested to cause, at least partly, the cognitive deficits observed in patients with Alzheimer's disease. Impaired cortical cholinergic neurotransmission may also contribute to beta-amyloid plaque pathology in Alzheimer's disease by affecting expression and processing of the beta-amyloid precursor protein (APP). Vice versa, low level of soluble beta-amyloid has been observed to inhibit cholinergic synaptic function. Deposition of beta-amyloid plaques in Alzheimer's disease is also accompanied by a significant plaque-associated glial up-regulation of interleukin-1, which has been attributed to affect expression and metabolism of APP and to interfere with cholinergic transmission. Understanding the molecular mechanisms underlying the interrelationship between cortical cholinergic dysfunction, beta-amyloid formation and deposition, as well as local inflammatory upregulation, would allow to derive potential treatment strategies to pharmacologically intervene in the disease-causing signaling cascade.

Muscarinic acetylcholine receptor inhibition in transgenic Alzheimer-like Tg2576 mice by scopolamine favours the amyloidogenic route of processing of amyloid precursor protein

The molecular mechanisms of the interrelationship between cholinergic neurotransmission, processing of amyloid precursor protein (APP) and beta-amyloid (Abeta) production in vivo are still less understood. To reveal any effect of cholinergic dysfunction on APP processing in vivo, 11-month-old transgenic Tg2576 mice with Abeta plaque pathology received intraperitoneal injections of scopolamine at a daily dosage of 2mg/kg body weight for 14 days in order to suppress cortical cholinergic transmission by chronic inhibition of muscarinic acetylcholine receptors. Scopolamine treatment of transgenic Tg2576 mice resulted in increased levels of fibrillar Abeta(1-40) and Abeta(1-42), while the soluble, SDS-extractable Abeta level remained unchanged as compared to vehicle-injected Tg2576 mice. alpha-Secretase activity determined in cortical tissue from scopolamine-treated Tg2576 mice was lower by about 30% as compared to that assayed in control mice, while beta-secretase activity and BACE1 protein expression appeared unaffected by scopolamine treatment. The amount of sAPPalpha, the product secreted by alpha-secretase-mediated APP cleavage, and the unprocessed APP were assayed in the soluble and membrane fraction, respectively, of cortical tissue preparations from treated and control mice by Western blotting. Using the anti antibody 6E10 which specifically labels human sAPPalpha and full length APP in transgenic Tg2576, an enhanced APP level was detected in the membrane fraction from treated mice as compared to controls, while in the soluble fraction scopolamine treatment did not affect the protein level of sAPPalpha. These data indicate an accumulation of APP in cortical membrane fraction in scopolamine-treated Tg2576 mice presumably due to the decreased level of alpha-secretase-mediated APP cleavage, and further suggest that chronic suppression of cortical muscarinic cholinergic transmission may alter the balance between alpha- and beta-secretory APP processing by favouring the amyloidogenic route.

Spatial and temporal control of age-related APP processing in genomic-based beta-secretase transgenic mice

Genetic mutations associated with Alzheimer's disease (AD) in the Amyloid Precursor Protein (APP) gene specifically alter the production of the APP processing product, amyloid-beta (Abeta) peptide, generated by beta- and gamma-secretases. The accumulation and deposition of Abeta is hypothesized to cause AD pathogenesis, leading to the debilitating neurological deficits observed in AD patients. However, it is unclear how processing of APP to generate Abeta corresponds with the age-dependent pattern of brain-regional neurodegeneration common in AD. We have previously shown that overexpression of BACE1, the primary beta-secretase gene, in mice expressing an AD mutant form of APP leads to significantly elevated regional Abeta levels, which coincide with the regional pattern of Abeta deposition. In the current study, we have used our genomic-based beta-secretase transgenic mice to determine how BACE1 regulates the spatial and temporal pattern of Abeta production throughout post-natal development. Specifically, we observed unique differences in the brain-regional expression pattern between neonatal and adult BACE1 transgenic mice. These alterations in the BACE1 expression profile directly corresponds with age-related differences in regional Abeta production and deposition. These studies indicate that modulation of BACE1 expression leads to dramatic alterations in APP processing and AD-like neuropathology. Furthermore, our studies provide further evidence that BACE1 plays a major role in the regulation of the APP processing pathway, influencing the age-dependent onset of AD pathogenesis.

Down-regulation of muscarinic acetylcholine receptor M2 adversely affects the expression of Alzheimer's disease-relevant genes and proteins

Beta-amyloid peptides play a major role in the pathogenesis of Alzheimer's disease (AD). Therefore, preventing beta-amyloid formation by inhibition of the beta site amyloid precursor protein-cleaving enzyme (BACE) 1 is considered as a potential strategy to treat AD. Cholinergic mechanisms have been shown to control amyloid precursor protein processing and the number of muscarinic M2-acetylcholine receptors is decreased in brain regions of patients with AD enriched with senile plaques. Therefore, the present study investigates the effect of this M2 muscarinic receptor down-regulation by siRNA on total gene expression and on regulation of BACE1 in particular in SK-SH-SY5Y cells. This model system was used for microarray analysis after carbachol stimulation of siRNA-treated cells compared with carbachol stimulated, non-siRNA-treated cells. The same model system was used to elucidate changes at the protein level by using two-dimensional gels followed by Matrix Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF) analysis. Taken together, the results indicate that the M2 acetylcholine receptor down-regulation in brains of patients with AD has important effects on the expression of several genes and proteins with major functions in the pathology of AD. This includes beta-secretase BACE1 as well as several modulators of the tau protein and other AD-relevant genes and proteins. Moreover, most of these genes and proteins are adversely affected against the background of AD.