Non-amyloidogenic Alpha-secretase Research Articles

HIV-Associated Insults Modulate ADAM10 and Its Regulator Sirtuin1 in an NMDA Receptor-Dependent Manner.

Neurologic deficits associated with human immunodeficiency virus (HIV) infection impact about 50% of persons with HIV (PWH). These disorders, termed HIV-associated neurocognitive disorders (HAND), possess neuropathologic similarities to Alzheimer’s disease (AD), including intra- and extracellular amyloid-beta (Aβ) peptide aggregates. Aβ peptide is produced through cleavage of the amyloid precursor protein (APP) by the beta secretase BACE1. However, this is precluded by cleavage of APP by the non-amyloidogenic alpha secretase, ADAM10. Previous studies have found that BACE1 expression was increased in the CNS of PWH with HAND as well as animal models of HAND. Further, BACE1 contributed to neurotoxicity. Yet in in vitro models, the role of ADAM10 and its potential regulatory mechanisms had not been examined. To address this, primary rat cortical neurons were treated with supernatants from HIV-infected human macrophages (HIV/MDMs). We found that HIV/MDMs decreased levels of both ADAM10 and Sirtuin1 (SIRT1), a regulator of ADAM10 that is implicated in aging and in AD. Both decreases were blocked with NMDA receptor antagonists, and treatment with NMDA was sufficient to induce reduction in ADAM10 and SIRT1 protein levels. Furthermore, decreases in SIRT1 protein levels were observed at an earlier time point than the decreases in ADAM10 protein levels, and the reduction in SIRT1 was reversed by proteasome inhibitor MG132. This study indicates that HIV-associated insults, particularly excitotoxicity, contribute to changes of APP secretases by downregulating levels of ADAM10 and its regulator.

Propargylamine Containing Compounds as Modulators of Proteolytic Cleavage of Amyloid Protein Precursor: Involvement of MAPK and PKC Activation

The anti-Parkinsonian, irreversible, selective monoamine oxidase (MAO)-B inhibitors, selegiline (deprenyl, (R)-N-methyl-N-(1-phenylpropan-2-yl) prop-2-yn-1-amine) and rasagiline (Azilect, N-propargyl-1(R)-aminoindan), have been proven to possess neuroprotective/neurorestorative activities in cell cultures and animal models of neurodegenerative diseases. Structure-activity studies provide evidence that neuroprotection is associated with some intrinsic pharmacological action of the propargylamine moiety in these drugs. This indication and recent therapeutic approaches, entailing new drug candidates possessing diverse pharmacological properties and acting on multiple targets, have stimulated the development of two multifunctional chimeric propargylamine-derivatives: 1) ladostigil (TV3326, [(N-propargyl-(3R) 1-(R)-aminoindan-5yl)-ethyl methyl carbamate)], which combines the pharmacophore of rasagiline, with the carbamate moiety of the cholinesterase inhibitor rivastigmine, as a potential treatment for Alzheimer's disease and Lewy body disease; and 2) M30 5-[(N-methyl-N-propargylaminomethyl)-8-hydroxyquinoline], where the propargylamine moiety of rasagiline was embedded onto the backbone of the neuroprotective and brain permeable iron chelator 8-hydroxyquinoline-derivative, VK28 as a potential treatment for various neurodegenerative disorders. Both multifunctional propargylamine-derivatives were found to possess neuroprotective and anti-apoptotic properties. An additional and new neuroprotective effect, shared by the propargylamine-derivative compounds, is related to their ability to regulate the processing of amyloid-beta protein precursor (AbetaPP) by the non-amyloidogenic alpha-secretase pathway. This effect was shown to involve activation of p42/44 mitogen-activated protein kinase (MAPK) and protein kinase C (PKC) signaling pathway. This review will summarize and discuss current research, focused on the effect of propargylamine-related derivatives on the proteolytic processing of AbetaPP and signal transduction mechanisms.

A Natural Squamosamide Derivative FLZ Reduces Amyloid-β Production by Increasing Non-Amyloidogenic AβPP Processing

Substantial evidence supports a central role of Abeta in the pathogenesis of Alzheimer's disease (AD). We have demonstrated that FLZ, a synthetic cyclic analogue of natural squamosamide, exhibits neuroprotective actions in cells and mouse models, suggesting future investigation of FLZ as a candidate compound for the treatment of AD. In this study, we found that the production of amyloid-beta (Abeta) was reduced by FLZ in Abeta-expressing neuroblastoma cells, and correlated with an increase in the soluble alpha-secretase derived fragment of the amyloid-beta protein precursor (sAbetaPPalpha) in the medium. Moreover, the active form of ADAM10 and AbetaPP were elevated at the cell surface of FLZ-treated cells, consistent with an enhanced co-localization of ADAM10 and AbetaPP on the membrane. Pretreatment with brefeldin, a protein trafficking inhibitor, blocked FLZ-induced translocation of ADAM10 to the cell surface and release of sAbetaPPalpha to the culture medium. Furthermore, oral administration of FLZ to APPswe/PS1 transgenic mice significantly reduced the levels of Abeta, paralleling with activation of ADAM10, in the hippocampus. In silico prediction indicates that the structure of FLZ is compatible with the drug-like rules for absorption and permeability. These findings suggest that FLZ reduces Abeta production by promoting AbetaPP non-amyloidogenic alpha-secretase processing. As such, FLZ may have therapeutic potential for the treatment of AD.

Regulation of Forkhead Transcription Factor FoxO3a Contributes to Calorie Restriction‐induced Prevention of Alzheimer's Disease‐type Amyloid Neuropathology and Spatial Memory Deterioration

Forkhead transcription factor FoxO3a, also known as DAF-16 in Caenorhabditis elegans, is a key regulator of the insulin receptor (IR)/insulin-like growth factor-I signaling pathway mediated extension of life span in worms and yeast. In this study, we report that calorie restriction (CR)-mediated activation of the IR signaling pathway leads to hyperphosphorylation of FoxO3a transcription factor and, consequently, its exclusion from the nucleus. This inactivation of FoxO3a activity is correlated with attenuation of Alzheimer's disease (AD)-type amyloid neuropathology and with preservation of spatial reference memory in the Tg2576 mouse model of AD. Further, in vitro studies reveal that exogenous expression of viral, triple-mutant, constitutively active FoxO3a resulting in increased nuclear FoxO3a activity in primary neuron cultures derived from Tg2576 mouse embryos, causally promotes AD amyloid-beta peptide (Abeta) levels by inhibiting nonamyloidogenic alpha-secretase activity, indicating the existence of an inverse correlation between FoxO3a activity and cerebral Abeta amyloidosis. Moreover, we report for the first time that the exclusion of the FoxO3a transcription factor from the nucleus in combination with inhibition of nuclear FoxO3a activity by SIRT1-mediated deacetylation in response to CR is a mechanism resulting in the repression of Rho-associated protein kinase-1 gene expression, thereby activating nonamyloidogenic alpha-secretase processing of the amyloid precursor protein and lowering Abeta generation. This study provides a novel metabolic pathway for prevention and/or treatment of AD.

In Vivo β-Secretase 1 Inhibition Leads to Brain Aβ Lowering and Increased α-Secretase Processing of Amyloid Precursor Protein without Effect on Neuregulin-1

beta-Secretase (BACE) cleavage of amyloid precursor protein (APP) is one of the first steps in the production of amyloid beta peptide Abeta42, the putative neurotoxic species in Alzheimer's disease. Recent studies have shown that BACE1 knockdown leads to hypomyelination, putatively caused by a decline in neuregulin (NRG)-1 processing. In this study, we have tested a potent cell-permeable BACE1 inhibitor (IC(50) approximately 30 nM) by administering it directly into the lateral ventricles of mice, expressing human wild-type (WT)-APP, to determine the consequences of BACE1 inhibition on brain APP and NRG-1 processing. BACE1 inhibition, in vivo, led to a significant dose- and time-dependent lowering of brain Abeta40 and Abeta42. BACE1 inhibition also led to a robust brain secreted (s)APPbeta lowering that was accompanied by an increase in brain sAPPalpha levels. Although an increase in full-length NRG-1 levels was evident in 15-day-old BACE1 homozygous knockout (KO) (-/-) mice, in agreement with previous studies, this effect was also observed in 15-day-old heterozygous (+/-) mice, but it was not evident in 30-day-old and 2-year-old BACE1 KO (-/-) mice. Thus, BACE1 knockdown led to a transient decrease in NRG-1 processing in mice. Pharmacological inhibition of BACE1 in adult mice, which led to significant Abeta lowering, was without any significant effect on brain NRG-1 processing. Taken together, these results suggest that BACE1 is the major beta-site cleavage enzyme for APP and that its inhibition can lower brain Abeta and redirect APP processing via the potentially nonamyloidogenic alpha-secretase pathway, without significantly altering NRG-1 processing.

Apolipoprotein E genotype and alpha‐tocopherol modulate amyloid precursor protein metabolism and cell cycle regulation

Apolipoprotein E4 (apoE4) genotype is associated with an increased risk for Alzheimer's disease (AD). This is thought to be in part attributable to an impact of apoE genotype on the processing of the transmembrane amyloid precursor protein (APP) thereby contributing to amyloid beta peptide formation in apoE4 carriers, which is a primary patho-physiological feature of AD. As apoE and alpha-tocopherol (alpha-toc) have been shown to modulate membrane bilayer properties and hippocampal gene expression, we studied the effect of apoE genotype on APP metabolism and cell cycle regulation in response to dietary alpha-toc. ApoE3 and apoE4 transgenic mice were fed a diet low (VE) or high (+VE) in vitamin E (3 and 235 mg alpha-toc/kg diet, respectively) for 12 weeks. Cholesterol levels and membrane fluidity were not different in synaptosomal plasma membranes isolated from brains of apoE3 and apoE4 mice (-VE and +VE). Non-amyloidogenic alpha-secretase mRNA concentration and activity were significantly higher in brains of apoE3 relative to apoE4 mice irrespective of the dietary alpha-toc supply, while amyloidogenic beta-secretase and gamma-secretase remained unchanged. Relative mRNA concentration of cell cycle related proteins were modulated differentially by dietary alpha-toc supplementation in apoE3 and apoE4 mice, suggesting genotype-dependent signalling effects on cell cycle regulation.

Mitochondrial Dysfunction in Aging and Alzheimer's Disease: Strategies to Protect Neurons

Recent structural and functional studies of mitochondria have revealed that abnormalities in mitochondria may lead to mitochondrial dysfunction in aged individuals and those with neurodegenerative diseases, including Alzheimer's disease (AD). Molecular, cellular, and biochemical studies of animal models of aging and AD have provided compelling evidence that mitochondria are involved in AD development and progression. Further, a role for mitochondrial dysfunction in AD is supported by studies of neurons from autopsy specimens of patients with AD, transgenic AD mice, and neuronal cells expressing human AD mutation, which have revealed that amyloid beta (Abeta) enters mitochondria early in the disease process and disrupts the electron-transport chain, generates reactive oxygen species, and inhibits the production of cellular ATP, which in turn prevents neurons from functioning normally. Although AD researchers are actively involved in understanding Abeta toxicity and trying to develop strategies to reduce Abeta toxicity, one route they have yet to take is to investigate the molecules that activate nonamyloidogenic alpha-secretase activity that may reduce Abeta production and toxicity. In addition, it may be worthwhile to develop mitochondrially targeted antioxidants to treat AD. This article discusses critical issues of mitochondria causing dysfunction in aging and AD and discusses the strategies to protect neurons caused by mitochondrial dysfunction.

A multifunctional, neuroprotective drug, ladostigil (TV3326), regulates holo‐APP translation and processing

The recent therapeutic approach in which drug candidates are designed to possess diverse pharmacological properties and act on multiple targets has stimulated the development of the bifunctional drug ladostigil (TV3326) [(N-propargyl-(3R) aminoindan-5yl)-ethyl methyl carbamate]. Ladostigil combines the neuroprotective effects of the antiparkinson drug rasagiline, a selective monoamine oxidase (MAO)-B inhibitor, with the cholinesterase (ChE) inhibitory activity of rivastigmine in a single molecule, as a potential treatment for Alzheimer's disease (AD) and Lewy Body disease. Here, we assessed the dual effects of lodostigil in terms of the molecular mechanism of neuroprotection and amyloid precursor protein (APP) regulation/processing by using an apoptotic model of neuroblastoma SK-N-SH cells. Ladostigil dose-dependently decreased cell death via inhibition of the cleavage and prevention of caspase-3 activation (IC50=1.05 microM) through a mechanism related to regulation of the Bcl-2 family proteins, which resulted in reduced levels of Bad and Bax and induced levels of Bcl-2 gene and protein expression. We have also followed APP regulation/processing and found that ladostigil markedly decreased apoptotic-induced levels of holo-APP protein without altering APP mRNA levels, suggesting a posttranscriptional mechanism. In addition, the drug-elevated phosphorylated protein kinase C (pPKC) levels and stimulated the release of the nonamyloidogenic alpha-secretase proteolytic pathway. Similar to ladostigil, its S-isomer, TV3279, which is a ChE inhibitor but lacks MAO inhibitory activity, exerted neuroprotective properties and regulated APP processing, indicating that these effects are independent of MAO inhibition.

Effects of huperzine A on amyloid precursor protein processing and β‐amyloid generation in human embryonic kidney 293 APP Swedish mutant cells

The amyloid precursor protein (APP) is cleaved enzymatically by nonamyloidogenic and amyloidogenic pathways. alpha-Secretase (alpha-secretase), cleaves APP within the beta-amyloid (Abeta) sequence, resulting in the release of a secreted fragment of APP (alphaAPPs) and precluding Abeta generation. In this study, we investigated the effects of an acetylcholinesterase inhibitor, huperzine A (Hup A), on APP processing and Abeta generation in human embryonic kidney 293 cells transfected with human APP bearing the Swedish mutation (HEK293 APPsw). Hup A dose dependently (0-10 microM) increased alphaAPPs release and membrane-coupled APP CTF-C83, suggesting increased APP metabolism toward the nonamyloidogenic alpha-secretase pathway. The metalloprotease inhibitor TAPI-2 inhibited the Hup A-induced increase in alphaAPPs release, further suggesting a modulatory effect of Hup A on alpha-secretase activity. The synthesis of full-length APP and cell viability were unchanged after Hup A incubation, whereas the level of Abeta(Total) was significantly decreased, suggesting an inhibitory effect of Hup A on Abeta production. Hup A-induced alphaAPPs release was significantly reduced by the protein kinase C (PKC) inhibitors GF109203X and Calphostin C. These data, together with the finding that the PKCalpha level was enhanced prior to the increase of alphaAPPs secretion, indicate that PKC may be involved in Hup A-induced alphaAPPs secretion by HEK293 APPsw cells. Our data suggest alternative pharmacological mechanisms of Hup A relevant to the treatment of Alzheimer's disease.

Regulation of Bcl‐2 family proteins, neurotrophic factors, and APP processing in the neurorescue activity of propargylamine

The anti-Parkinson drug, rasagiline (N-propargyl-(1R)-aminoindan) promotes neuronal survival, via neuroprotective activity related to its propargyl moiety (propargylamine). We have investigated the neurorescue effects of propargylamine, in a progressive neuronal death model, induced by long-term serum deprivation in human SH-SY5Y neuroblastoma cells. Propargylamine (0.1-10 microM) dose-dependently reduced the levels of the early apoptosis-associated phosphorylated protein, H2A-X (ser 139), as well as decreased the cleavage of caspase-3 and its substrate poly-ADP ribose polymerase (PARP). In addition, the compound markedly reversed the apoptotic effects induced by long-term serum withdrawal, including down-regulation of the antiapoptotic protein, Bcl-2, as well as up-regulation of the proapoptotic proteins, Bax, Bad, and Bim. Real-time RT-PCR demonstrated that propargylamine elevated gene expression levels of Bcl-2, and the neurotrophic factors glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) and reduced Bax gene expression. Serum deprivation increased mRNA and protein levels of holo-amyloid precursor protein (APP), which was markedly decreased by propargylamine. This was accompanied by inducing the release of the nonamyloidogenic alpha-secretase form of soluble APP (sAPPalpha) into the medium. Similar effects on cell survival and APP regulation/processing were demonstrated for rasagiline. These results indicate that both rasagiline and propargylamine possess neurorescue activity, associated with regulation of Bcl-2 family proteins, neurotrophic factors, and APP metabolism.

Amyloid processing and signal transduction properties of antiparkinson-antialzheimer neuroprotective drugs rasagiline and TV3326.

Two novel neuroprotective cholinesterase (ChE) inhibitors, TV3326 and TV3279 [(N-propargyl-(3R) and (3S) aminoindan-5-yl)-ethyl methyl carbamate], respectively were derived from rasagiline, for the treatment of Alzheimer's disease (AD). TV3326 also inhibits monoamine oxidase (MAO)-A and B, while its S-isomer, TV3279, lacks MAO-inhibitory activity. The actions of these drugs in the regulation of the amyloid precursor protein (APP) processing using rat PC12 and human SH-SY5Y neuroblastoma cells were examined. Both isomers stimulated the release of the non-amyloidogenic alpha-secretase form of soluble APP (sAPPalpha) from these cell lines. The increases in sAPPalpha, induced by TV3326 and TV3279, were dose-dependent (0.1-100 micro M) and blocked by the hydroxamic acid-based metalloprotease inhibitor, Ro31-9790, suggesting mediation via alpha-secretase activity. Using several signal transduction inhibitors, the involvement of protein kinase C (PKC), mitogen-activated protein (MAP) kinase, and tyrosine kinase-dependent pathways in the enhancement of sAPPalpha release by TV3326 and TV3279 was identified. In addition, both drugs directly induced the phosphorylation of p44 and p42 MAP kinase, which was abolished by the specific inhibitors of MAP kinase activation, PD98059 and U0126. These data suggest a novel pharmacological mechanism, whereby these ChE inhibitors regulate the secretary processes of APP via activation of the MAP kinase pathway.

Non-steroidal Anti-inflammatory Drugs Stimulate Secretion of Non-amyloidogenic Precursor Protein

Chronic inflammatory processes are associated with the pathophysiology of Alzheimer's disease (AD), and it has been proposed that treatment with non-steroidal anti-inflammatory drugs (NSAIDs) reduces the risk for AD. Here we report that various NSAIDs, such as the cyclooxygenase inhibitors, nimesulide, ibuprofen and indomethacin, as well as thalidomide (Thal) and its non-teratogenic analogue, supidimide, significantly stimulated the secretion of the non-amyloidogenic alpha-secretase form of the soluble amyloid precursor protein (sAPP alpha) into the conditioned media of SH-SY5Y neuroblastoma and PC12 cells. These NSAIDs markedly reduced the levels of the cellular APP holoprotein, further accelerating non-amyloidogenic processes. sAPP alpha release, induced by nimesulide and Thal, was modulated by inhibitors of protein kinase C and Erk mitogen-activated protein (MAP) kinase. Furthermore, in results complementary to the inhibitor studies, we show for the first time that NSAIDs can activate the Erk MAP kinase signaling cascade, thus identifying a novel pharmacology mechanism of NSAIDs. Our findings suggest that NSAIDs and Thal might prove useful to favor non-amyloidogenic APP processing by enhancing alpha-secretase activity, thereby reducing the formation of amyloidogenic derivatives, and therefore are of potential therapeutic value in AD.

Generation of C-terminally truncated amyloid-beta peptides is dependent on gamma-secretase activity.

Aberrant production of amyloid-beta peptides by processing of the beta-amyloid precursor protein leads to the formation of characteristic extracellular protein deposits which are thought to be the cause of Alzheimer's disease. Therefore, inhibiting the key enzymes responsible for amyloid-beta peptide generation, beta- and gamma-secretase may offer an opportunity to intervene with the progression of the disease. In human brain and cell culture systems a heterogeneous population of amyloid-beta peptides with various truncations is detected and at present, it is unclear how they are produced. We have used a combination of surface enhanced laser desorption/ionization time-of-flight mass spectrometry (SELDI-TOF MS) and a specific inhibitor of gamma-secretase to investigate whether the production of all amyloid-beta peptide species requires the action of gamma-secretase. Using this approach, we demonstrate that the production of all truncated amyloid-beta peptides except those released by the action of the nonamyloidogenic alpha-secretase enzyme or potentially beta-site betaAPP cleaving enzyme 2 depends on gamma-secretase activity. This indicates that none of these peptides are generated by a separate enzyme entity and a specific inhibitor of the gamma-secretase enzyme should havethe potential to block the generation of all amyloidogenicpeptides. Furthermore in the presence of gamma-secretase inhibitors, the observation of increased cleavage of the membrane-bound betaAPP C-terminal fragment C99 by alpha-secretase suggests that during its trafficking C99 encounters compartments in which alpha-secretase activity resides.

Intracellular Aβ1-42 Aggregates Stimulate the Accumulation of Stable, Insoluble Amyloidogenic Fragments of the Amyloid Precursor Protein in Transfected Cells

We have analyzed the effect of internalized amyloid beta-protein (A beta) 1-42 aggregates on the metabolism of the amyloid precursor protein (APP) in stably transfected 293 cells. The amount of potentially amyloidogenic fragments of APP immunoprecipitated by anti-carboxyl-terminal APP and anti-A beta antibodies is dramatically enhanced by the treatment of the cells with A beta 1-42, which is resistant to degradation, but not A beta 1-28, which does not accumulate in cells. This accumulation of amyloidogenic carboxyl-terminal fragments is specific, since there is relatively little effect of A beta 1-42 on the amount of the nonamyloidogenic alpha-secretase carboxyl-terminal fragment. The amyloidogenic fragments accumulate in the same nonionic detergent-insoluble fraction of the cell that contains the internalized A beta 1-42. Western analysis indicates that a subset of the amyloidogenic fragments react with antibodies that recognize a conformation of A beta that is specifically associated with aggregated forms of A beta, suggesting that the adoption of this aggregation-related conformation may be an early event which precedes the final processing that produces A beta. Pulse-chase analysis of the [35S]Met-labeled 16-kDa amyloidogenic fragment indicates that it is relatively stable in A beta 1-42-treated cells, with a half-life of approximately 50 h. This fragment is degraded with a half-life of 30 min in control cells treated with A beta 1-28. In contrast, the turnover of the nonamyloidogenic alpha-secretase product is not significantly altered by the presence of A beta 1-42. The continuous uptake of A beta 1-42 from the medium is not required for the stimulation of amyloidogenic fragment accumulation, suggesting that the presence of intracellular A beta 1-42 aggregates establishes a new pathway for APP catabolism in cells which leads to the long term stability of the fragments. If these amyloidogenic fragments of APP ultimately give rise to A beta, then the production of A beta may be an autocatalytic, "runaway" process in cells containing A beta 1-42 nuclei. It is conceivable that the accumulation of insoluble APP and amyloidogenic fragments of APP in response to A beta 1-42 aggregates may mimic the pathophysiology of dystrophic neurites, where the accumulation of intracellular APP and APP fragments has been documented by immunohistochemistry.