Abstract: Alzheimer's disease (AD) is a progressive and degenerative disorder pathophysiologically characterized by the accumulation of beta-amyloid peptides (Aβ) in the brain. Aβ is indicated to be the primary agent in the pathogenesis of AD. Aβ is generated from the amyloid precursor protein (APP) via two proteolytic enzymes, β- and γ- secretases. α-secretase conducts an alternative proteolytic cleavage that prevents Aβ production and accumulation. Elevating levels of α-secretase cleavage, therefore, is a potential therapeutic strategy to treat AD.
Alzheimer’s disease (AD) is a progressive and degenerative disorder clinically characterized by dementia that inevitably leads to incapacitation and death. AD is the most common form of dementia and affects about 40% of individuals aged 85 or over. More than 5 million Americans now have AD. There is currently no cure to stop the disease progression. Advanced age and a family history of the disease are the largest risk factors for developing AD. Genetically, mutation in three genes (APP, PSEN-1, and PSEN-2) and polymorphisms in one gene, APOE, have been confirmed to be involved in AD (Tanzi and Bertram, 2001). Recent meta-analysis results suggest that numerous genes (~25) contribute a small yet significant effect to the risk of developing AD. In addition to age and genetics, environmental factors such as diet, smoking, and education also modulate the risk of developing late onset AD.
The underlying mechanism of AD pathogenesis is still not completely understood. At the microscopic level, AD is characterized by two obvious pathological hallmarks: (1) amyloid plaques, which are primarily composed of the 39-43 amino-acid Aβ peptide; and (2) neurofibrillary tangles, which are paired helical filaments of hyperphosphorylated tau protein. A large amount of genetic, cell biology, and biochemical evidence supports the amyloid cascade hypothesis of AD pathogenesis. This hypothesis states that Aβ production and accumulation is the primary cause of AD. Soluble Aβ affects synaptic functions. Accumulated Aβ induces an inflammatory response followed by neuritic injury, hyperphosphorylation of tau protein, and formation of fibrillary tangles, leading ultimately to neuronal dysfunction and cell death (Gandy, 2005; Selkoe, 2001; Tanzi and Bertram, 2001). The Aβ peptide is generated from an integral type I membrane protein called β-amyloid precursor protein (APP). Thus, genes and proteins that can modulate APP metabolism and, specifically, decrease Aβ production and accumulation are currently envisioned as the desired targets for AD therapeutics.
APP Metabolism — Amyloidogenic and Non-amyloidogenic Pathways
APP is a transmembrane protein that can undergo sequential proteolytic cleavage (Figure 1). Depending on whether Aβ is eventually generated, the pathway is termed amyloidogenic or non-amyloidogenic. In the amyloidogenic pathway, the protease that first cleaves APP is β-secretase. This cleavage occurs in the APP lumenal domain at the N-terminus of the Aβ peptide sequence. Cleavage produces a soluble N-terminal APP fragment (or sAPPβ) and a transmembrane C-terminal fragment (β-CTF). β-CTF is then cleaved within the membrane by γ-secretase and generates Aβ and APP intracellular domain (AICD). Depending on the γ-secretase cleavage site, there are two main Aβ species generated: Aβ40 and Aβ42. Aβ42 is more hydrophobic and aggregates more readily than Aβ40. β-secretase cleavage of APP is carried out by the BACE1 protein, which is a transmembrane protein of the pepsin family. γ-secretase is comprised of at least four membrane proteins, presenilin, presenilin enhancer-2, nicastrin, and anterior pharynx defective 1.
In the non-amyloidogenic pathway, the protease that first cleaves APP is α-secretase. This cleavage occurs in the middle of the Aβ region and yields a soluble N-terminal APPα fragment (sAPPα) and a transmembrane C-terminal fragment (α-CTF). sAPPα has neurotrophic and neuroprotective functions. Similar to β-CTF, α-CTF is then cleaved by γ-secretase to generate a 23-25 amino-acid peptide termed p3, instead of Aβ, and AICD. Three enzymes have been shown to harbor α-secretase activity: ADAM9, ADAM10, and ADAM17. They all belong to the family of A Disintegrin And Metalloprotease (ADAM) domain containing proteases, which are membrane-bound cell surface glycoproteins and have been implicated in cell adhesion, protein ectodomain shedding, matrix protein degradation, and cell fusion.
Inhibition of amyloidogenic processing is an obvious therapeutic approach to treating AD. Compounds that inhibit β- or γ-secretase are not yet approved but are in clinical trials currently. Other therapeutic approaches that target Aβ production and accumulation are also under active investigation, they include a-secretase stimulation, immunotherapy, selective Aβ42-lowering agents (tarenflurbil), inhibitors of amyloid aggregation (tramiprosate), and cholesterol-lowering agent (statins). Among these strategies, stimulating α-secretase is the only direct gain-of-function strategy and appears to be the safest strategy because sAPPα is indicated to be neurotrophic and neuroprotective. Our group and some other groups showed that ADAM9, ADAM10, and ADAM17 are constitutively expressed; in addition, the activities of ADAM10 and ADAM17 can be stimulated by a number of signal transduction pathways including protein kinase C (PKC), phospholipase A2 (PLA2), and muscarinic acetylcholine receptors (mAChRs). Therefore genes and pathways that stimulate a-secretase activities should be considered good therapeutic targets.
Currently, AD associated cognitive decline can be temporarily treated by drugs that increase acetylcholine levels at the synapse (Lyketsos et al., 2006). These drugs, tacrine (Cognex), donepezil (Aricept), galantamine (Reminyl), and rivastigmine (Exelon), preserve the activity of cholinergic neurons. Another approved drug, Memantine, protects glutamatergic neurons from hypo- or hyper-activation induced damage. At the time these drugs were approved it was thought that these drugs treat the symptoms of the disease and not Aβ accumulation. Recent work however suggests that some of these drugs stimulate APP α-secretase processing.
Here we review the current progress in identifying genes, pathways, and compounds that can stimulate a-secretase processing of APP.
Genes and Proteins That Can Modulate α-Secretases Activity
The results of genetic linkage and association studies have been very useful in identifying the genes, proteins, and pathways involved in AD pathogenesis. These approaches have also identified a-secretase modulators. Specifically, N-arginine dibasic convertase (NRDc or nardilysin) is a peptidase that hydrolyzes peptide substrates at arginine-arginine or arginine-lysine sequences. The gene Nardilysin is located on chromosome 1p32, a genomic region implicated to harbor an AD locus from linkage studies (Blacker et al., 2003). The nardilysin protein directly enhances the a-secretase activity by regulating ADAMs, and decreases the generation of Aβ by using an unidentified mechanism. The nardilysin protein is expressed in adult heart, skeletal muscle, testis, and cortical neurons of the human brain.
CYP46A1 (cytochrome P450, family 46, subfamily a, polypeptide 1) is another gene that encodes a protein which can elevate α-secretase activity indirectly. The gene is located on the chromosome 12 in a region linked to AD, and polymorphisms within the gene have been associated with increased AD risk. CYP46A1 encodes the protein cholesterol 24-hydroxylase, which is expressed almost exclusively in the brain. It catalyzes cholesterol into 24S-hydroxycholesterol (24S-OHC), a major pathway to efflux excess cholesterol from the brain. This year, Famer et al. (2007) found that 24S-OHC also increases the a-secretase activity. It has been found that the brain 24S-OHC level is low in AD cases and the cholesterol level in the brain is high, accompanied by decreased a-secretase activities and increased b-secretase activities. Therefore, Famer et al. suggested that up-regulation of CYP46A1 could be a possible strategy for AD drug development.
Numerous investigations indicate that α-secretase activity can also be regulated by neurotransmitters and their receptors, particularly G-protein coupled receptors (GPCRs). GPCRs are a family of conserved proteins that contain seven transmembrane domains whose functions are diverse but primarily transmit signals induced by extracellular stimuli into the nuclei of the cells and induce transcription of target genes. GPCRs are of such clinical significance that nearly 40% of all prescription pharmaceuticals on the market target GPCRs (Filmore, 2004). Interestingly but not surprisingly it was recently found that APP processing is also controlled by GPCRs. Specifically, α-secretase activity can be modulated by GPCRs, like muscarinic acetylcholine receptors (mAChR), metabotropic glutamate receptor (mGluR), and purinergic receptor P2Y2. The underlying mechanism is still not completely elucidated but it appears that GPCRs sense the extracellular changes, including cholesterol levels, reactive oxygen species (ROS), hypoxia, and free nucleotides, and then transmit these signals to the interior of the cells and induce the transcription of kinases and a-secretase. For example after the M1 subtype of mAChRs, is activated by its agonist AF267B, protein kinase C (PKC) and phospholipase A2 (PLA2) are stimulated resulting in activation of ADAM17. In turn, ADAM17 promotes non-amyloidogenic processing of APP. Studies in animal models have also been demonstrated that AF267B can reduce Aβ deposition and rescue the cognitive deficits.
To systematically identify proteins that modulate APP proteolysis, Schobel et al. (2006) carried out an expression clone screening and identified several novel α-secretase modulators, namely, the endocytic proteins endophilin A1 and A3, palmitoyl-protein thioesterase 1 (PPT1), Numb-like, and the kinase MEKK2. Endophilin A3 demonstrated the strongest activity, which specifically increased sAPPα secretion and nonspecifically reduced the rate of APP endocytosis. Its encoding gene SH3GL3 (SH3-domain GRB2-like 3) is located on the chromosome 15q25, a region that is close to a genomic region (15q26) linked to AD. Endophilin A3 is preferentially expressed in the brain and testis. It is also suggested to be involved in the neuronal death and pathophysiology of Huntington’s disease.
Numerous lines of research have indicated that elevation of non-amyloidogenic pathway is a promising therapeutic strategy for the treatment of AD. Some potential therapeutics and lead compounds already exist. Specifically, a benzolactam derivative, TPPB [(2S,5S)-(E,E)-8-(5-(4-(trifluoromethyl)phenyl)-2,4-pentadienoylamino) benzolactam] is a novel α-secretase activator that stimulates the PKC pathway by an unidentified mechanism. It shifts APP processing towards the non-amyloidogenic pathway by increasing α-secretase activity. It also decreases β-secretase activity and Aβ40 levels. An extract from green tea, EGCG [Green tea polyphenol (-)-epigallocatechin-3-gallate], reduces brain Aβ levels by increasing a-secretase activity, specifically ADAM10, but does not significantly alter β- or γ-secretase activities. Other examples include acetylcholinesterase inhibitors (AChEIs), 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA) inhibitors (belonging to cholesterol lowing agents, like statins), and nonsteroidal anti-inflammatory drugs (NSAIDs). Besides their original primary effects, they all can stimulate α-secretase activity, and some may also inhibit β and/or γ activity as well. Donepezil (belonging to AChEIs) increases α-secretase secretion by promoting ADAM10 trafficking and/or maturation. NSAIDs and statins exert functions by modulating the isoprenoid pathway and activities of Rho-associated protein kinases (ROCKs), particularly ROCK1 activities, as well as by modulating inflammatory reactions and cytokine secretions.
It is well known that caloric restriction can expand life span, and recent findings suggest that caloric restriction alters APP processing. The underlying mechanism involves SIRT1, the NAD+-dependent sirtuin, whose encoding gene lies on chromosome 10q21, a genetic region linked to AD. It has been shown that caloric restriction, as well as treatment by resveratrol, an agonist of SIRT1 found in red wine, can dramatically increase the production of sAPPα, potentially by increasing ADAM10 levels.
These examples suggest that therapeutically targeting the α-secretase pathway is a viable option for treating AD. α-Secretases cleave other membrane protein substrates besides APP, therefore more research is needed to investigate the effects of α-secretase stimulation on these non-APP substrates. Detailed understanding of the underlying mechanisms of these α-secretase modulators of APP processing, as well as identifying novel and specific α-secretase modulators will open up novel avenues for the therapeutic intervention of AD.
References and Further Readings
Blacker D, Bertram L, Saunders AJ, Moscarillo TJ, Albert MS, Wiener H, Perry RT, Collins JS, Harrell LE, Go RC, et al. Results of a high-resolution genome screen of 437 Alzheimer’s disease families. Human Molecular Genetics 12:23-32, 2003.
Famer D, Meaney S, Mousavi M, Nordberg A, Bjorkhem I, Crisby M. Regulation of alpha- and beta-secretase activity by oxysterols: cerebrosterol stimulates processing of APP via the alpha-secretase pathway. Biochemical and Biophysical Research Communications 359:46-50, 2007.
Gandy S. The role of cerebral amyloid beta accumulation in common forms of Alzheimer disease. Journal of Clinical Investigation 115:1121-1129, 2005.
Lyketsos CG, Colenda CC, Beck C, Blank K, Doraiswamy MP, Kalunian DA, Yaffe K. Position statement of the American Association for Geriatric Psychiatry regarding principles of care for patients with dementia resulting from Alzheimer disease. American Journal of Geriatric Psychiatry 14:561-72, 2006.
Schobel S, Neumann S, Seed B, Lichtenthaler SF. Expression cloning screen for modifiers of amyloid precursor protein shedding. International Journal of Developmental Neuroscience 24:141-148, 2006.
Selkoe DJ. Alzheimer’s disease: genes, proteins, and therapy. Physiological Reviews 81:741-766, 2001.
Tanzi RE, Bertram L. New frontiers in Alzheimer’s disease genetics. Neuron 32:181-184, 2001.
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