Conventional protein kinase C (PKC)-alpha and novel PKC epsilon, but not -delta, increase the secretion of an N-terminal fragment of Alzheimer's disease amyloid precursor protein from PKC cDNA transfected 3Y1 fibroblasts

Mechanistic insight of the potential of geraniol against Alzheimer's disease

Background

Alzheimer’s disease (AD) as a neurodegenerative disease occupies 3/5–4/5 cases among patients with dementia, yet its pathogenetic mechanism remains unclear. Geraniol, on the other hand, is a well-known extract from essential oils of aromatic plants and has been proven that it has outstanding neuroprotective effects as well as ameliorating influence in memory impairment. Therefore, the present study aims to elucidate the potential of geraniol against AD by network pharmacology-based approach combined with molecular modeling study.

Materials and methods

Firstly, we evaluated the druggability of geraniol by ADME method. Then, we obtained the geraniol targets and AD-related targets from multiple open data sources. Afterward, we calculated the intersection through a Venn diagram to find common targets, and via Panther classification system to categorize them. In order to gain a macroscopic understanding of these common targets, we carried out GO terms and KEGG pathways enrichment analyses, according to which we constructed a compound–target–pathway–disease network. In addition, we built a preliminary PPI network which was further analyzed both functionally and topologically. Consequently, five hub targets were sorted out. Finally, we conducted molecular docking and molecular dynamic simulation to validate our findings.

Results

In the present study, the pharmacological properties of geraniol were assessed according to ADME and Lipinski’s rule, which demonstrate promising druggability. Then, from 10,972 AD-related targets and 33 geraniol targets, 29 common targets were identified, among which 38.1% of them are metabolite interconversion enzymes, 23.8% are protein modifying enzymes, 33.3% are transmembrane receptors, and the rest are transporters. Enrichment analyses hint that geraniol is involved in cholinergic synapse, serotonergic synapse, and neuroactive ligand–receptor interaction. We also built a preliminary PPI network to investigate the interplay between these targets and their extensive interactions. Then, by functionally clustering the preliminary PPI network, we gained a cluster of proteins which formed a subnetwork with score of 8.476, and 22 nodes. Its results of GO terms and KEGG pathways enrichment analyses once again suggests that geraniol actively participates in cholinergic synapse, serotonergic synapse, and neuroactive ligand–receptor interaction, which are believed to be strongly associated with AD pathogenesis. Besides, topological analyses of the preliminary PPI network helped find 5 hub targets (i.e., CHRM3, PRKCA, PRKCD, JAK1, JAK2). To verify their interaction with geraniol molecule, we conducted molecular docking, and found that CHRM3 possesses the highest affinity in binding, indicating that geraniol molecules are closely bound to each hub target, and CHRM3 may serve as a key target of geraniol against AD. It was then further confirmed by molecular dynamic simulation, the result of which supports our hypothesis.

Conclusion

The present study shares a mechanistic insight of the potential of geraniol against AD, giving a reference to future experimental studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40001-022-00699-8.

Protein Kinase C Involvement in Neuroprotective Effects of Thymol and Carvacrol Against Toxicity Induced by Amyloid-β in Rat Hippocampal Neurons

INTRODUCTION

We have reported that thymol and carvacrol can improve cognitive abilities in Alzheimer Disease (AD) rat models. However, the mechanism of their action is not yet fully understood. Recently, our in vitro results suggested that PC12 cell death induced by Aβ25-35 can be protected by thymol and carvacrol via Protein Kinase C (PKC) and Reactive Oxygen Species (ROS) pathways. So, we hypothesize that the mechanisms of thymol and carvacrol in improving the learning impairment in the AD rat model may be related to their effects on PKC. So, the activity of PKC and protein expression levels of PKCα were examined in the hippocampal cells of the AD rat model.

METHODS

To examine the thymol and carvacrol effects, we performed a behavioral test in AD rat models induced by Aβ_25-35 neurotoxicity. To access the underlying mechanism of the protective effects, western blotting was performed with antibodies against PKCα. We also measured the PKC activity assay by Elisa. Histopathological studies were carried out in the hippocampus with Hematoxylin and Eosin (H&E) staining.

RESULTS

The escape latency increased in Aβ-received rats compared to the control group, and thymol and carvacrol reversed this deficit. Furthermore, these compounds could enhance the PKC activity and increase the PKCα expression ratio. Moreover, H&E staining showed that Aβ caused shrinkage of the CA1 pyramidal neurons. However, thymol and carvacrol treatments could prevent this effect of Aβ peptides.

CONCLUSION

This study suggests that Amyloid-Beta (Aβ) results in memory decline and histochemical disturbances in the hippocampus. Moreover, these results revealed that thymol and carvacrol could have protective effects on cognition in AD-like models via PKC activation.

HIGHLIGHTS

Rat's ability to find the invisible platform in the Morris Water Maze (MWM) was impaired by Amyloid-Beta (Aβ) infusion in the hippocampus, while this effect was reversed by thymol or carvacrol administration.Aβ significantly downregulated the Protein Kinase C (PKC) activity in rats' hippocampus.Western blot analysis demonstrated that Aβ significantly reduced PKCα protein expression in AD rat model hippocampal cells.The expression ratio of PKCα was upregulated following the injection of thymol and carvacrol in rats.Injection of Aβ in the hippocampus resulted in histochemical disturbances in CA1 pyramidal neurons.Carvacrol and thymol can prevent several histological changes induced by Aβ.

PLAIN LANGUAGE SUMMARY

Alzheimer's disease is one of the most important brain diseases in which the learning and memory are impaired. One of the main causes of Alzheimer's disease is the presence of amyloid beta plaques in the neurons. Protein kinase C enzyme reduces amyloid production and accumulation in the brain. In the present study, we tested the possible effects of carvacrol and thymol in a rat model of Alzheimer's disease. Memory impairment was induced in adult rats by intra-cerebral infusion of amyloid β. One week later, the memory-impaired animals were treated with carvacrol and thymol. Finally, we tested their memory in a Morris water maze apparatus. Furthermore, their hippocampus was dissected and PKC activity and the neuronal injury was evaluated. Our findings exhibited that thymol and carvacrol improved rats' memory performance. In addition, thymol and carvacrol significantly increased PKC activity and prevented neuronal cell loss in the rat hippocampus. This study shows that thymol and carvacrol have beneficial effects on memory and cognitive function via PKC activation.

Cross-species genetic screens to identify kinase targets for APP reduction in Alzheimer's disease

An early hallmark of Alzheimer's disease is the accumulation of amyloid-β (Aβ), inspiring numerous therapeutic strategies targeting this peptide. An alternative approach is to destabilize the amyloid beta precursor protein (APP) from which Aβ is derived. We interrogated innate pathways governing APP stability using a siRNA screen for modifiers whose own reduction diminished APP in human cell lines and transgenic Drosophila. As proof of principle, we validated PKCβ-a known modifier identified by the screen-in an APP transgenic mouse model. PKCβ was genetically targeted using a novel adeno-associated virus shuttle vector to deliver microRNA-adapted shRNA via intracranial injection. In vivo reduction of PKCβ initially diminished APP and delayed plaque formation. Despite persistent PKCβ suppression, the effect on APP and amyloid diminished over time. Our study advances this approach for mining druggable modifiers of disease-associated proteins, while cautioning that prolonged in vivo validation may be needed to reveal emergent limitations on efficacy.

The Novel Perspectives of Adipokines on Brain Health

First seen as a fat-storage tissue, the adipose tissue is considered as a critical player in the endocrine system. Precisely, adipose tissue can produce an array of bioactive factors, including cytokines, lipids, and extracellular vesicles, which target various systemic organ systems to regulate metabolism, homeostasis, and immune response. The global effects of adipokines on metabolic events are well defined, but their impacts on brain function and pathology remain poorly defined. Receptors of adipokines are widely expressed in the brain. Mounting evidence has shown that leptin and adiponectin can cross the blood–brain barrier, while evidence for newly identified adipokines is limited. Significantly, adipocyte secretion is liable to nutritional and metabolic states, where defective circuitry, impaired neuroplasticity, and elevated neuroinflammation are symptomatic. Essentially, neurotrophic and anti-inflammatory properties of adipokines underlie their neuroprotective roles in neurodegenerative diseases. Besides, adipocyte-secreted lipids in the bloodstream can act endocrine on the distant organs. In this article, we have reviewed five adipokines (leptin, adiponectin, chemerin, apelin, visfatin) and two lipokines (palmitoleic acid and lysophosphatidic acid) on their roles involving in eating behavior, neurotrophic and neuroprotective factors in the brain. Understanding and regulating these adipokines can lead to novel therapeutic strategies to counteract metabolic associated eating disorders and neurodegenerative diseases, thus promote brain health.

Autotaxin⁻Lysophosphatidic Acid Signaling in Alzheimer's Disease

The brain contains various forms of lipids that are important for maintaining its structural integrity and regulating various signaling cascades. Autotaxin (ATX) is an ecto-nucleotide pyrophosphatase/phosphodiesterase-2 enzyme that hydrolyzes extracellular lysophospholipids into the lipid mediator lysophosphatidic acid (LPA). LPA is a major bioactive lipid which acts through G protein-coupled receptors (GPCRs) and plays an important role in mediating cellular signaling processes. The majority of synthesized LPA is derived from membrane phospholipids through the action of the secreted enzyme ATX. Both ATX and LPA are highly expressed in the central nervous system. Dysfunctional expression and activity of ATX with associated changes in LPA signaling have recently been implicated in the pathogenesis of Alzheimer’s disease (AD). This review focuses on the current understanding of LPA signaling, with emphasis on the importance of the autotaxin–lysophosphatidic acid (ATX–LPA) pathway and its alterations in AD and a brief note on future therapeutic applications based on ATX–LPA signaling.

Tyrosine Binding Protein Sites Regulate the Intracellular Trafficking and Processing of Amyloid Precursor Protein through a Novel Lysosome-Directed Pathway

The amyloid hypothesis posits that the production of β-amyloid (Aβ) aggregates leads to neurodegeneration and cognitive decline associated with AD. Aβ is produced by sequential cleavage of the amyloid precursor protein (APP) by β- and γ-secretase. While nascent APP is well known to transit to the endosomal/ lysosomal system via the cell surface, we have recently shown that APP can also traffic to lysosomes intracellularly via its interaction with AP-3. Because AP-3 interacts with cargo protein via interaction with tyrosine motifs, we mutated the three tyrosines motif in the cytoplasmic tail of APP. Here, we show that the YTSI motif interacts with AP-3, and phosphorylation of the serine in this motif disrupts the interaction and decreases APP trafficking to lysosomes. Furthermore, we show that phosphorylation at this motif can decrease the production of neurotoxic Aβ 42. This demonstrates that reducing APP trafficking to lysosomes may be a strategy to reduce Aβ 42 in Alzheimer’s disease.

Gain-of-function mutations in protein kinase Cα (PKCα) may promote synaptic defects in Alzheimer's disease

Alzheimer's disease (AD) is a progressive dementia disorder characterized by synaptic degeneration and amyloid-β (Aβ) accumulation in the brain. Through whole-genome sequencing of 1345 individuals from 410 families with late-onset AD (LOAD), we identified three highly penetrant variants in PRKCA, the gene that encodes protein kinase Cα (PKCα), in five of the families. All three variants linked with LOAD displayed increased catalytic activity relative to wild-type PKCα as assessed in live-cell imaging experiments using a genetically encoded PKC activity reporter. Deleting PRKCA in mice or adding PKC antagonists to mouse hippocampal slices infected with a virus expressing the Aβ precursor CT100 revealed that PKCα was required for the reduced synaptic activity caused by Aβ. In PRKCA(-/-) neurons expressing CT100, introduction of PKCα, but not PKCα lacking a PDZ interaction moiety, rescued synaptic depression, suggesting that a scaffolding interaction bringing PKCα to the synapse is required for its mediation of the effects of Aβ. Thus, enhanced PKCα activity may contribute to AD, possibly by mediating the actions of Aβ on synapses. In contrast, reduced PKCα activity is implicated in cancer. Hence, these findings reinforce the importance of maintaining a careful balance in the activity of this enzyme.

Lysophosphatidic acid induces increased BACE1 expression and Aβ formation

The abnormal production and accumulation of β-amyloid peptide (Aβ), which is produced from amyloid precursor protein (APP) by the sequential actions of β-secretase and γ-secretase, are thought to be the initial causative events in the development of Alzheimer's disease (AD). Accumulating evidence suggests that vascular factors play an important role in the pathogenesis of AD. Specifically, studies have suggested that one vascular factor in particular, oxidized low density lipoprotein (oxLDL), may play an important role in regulating Aβ formation in AD. However, the mechanism by which oxLDL modulates Aβ formation remains elusive. In this study, we report several new findings that provide biochemical evidence suggesting that the cardiovascular risk factor oxLDL may contribute to Alzheimer's disease by increasing Aβ production. First, we found that lysophosphatidic acid (LPA), the most bioactive component of oxLDL induces increased production of Aβ. Second, our data strongly indicate that LPA induces increased Aβ production via upregulating β-secretase expression. Third, our data strongly support the notion that different isoforms of protein kinase C (PKC) may play different roles in regulating APP processing. Specifically, most PKC members, such as PKCα, PKCβ, and PKCε, are implicated in regulating α-secretase-mediated APP processing; however, PKCδ, a member of the novel PKC subfamily, is involved in LPA-induced upregulation of β-secretase expression and Aβ production. These findings may contribute to a better understanding of the mechanisms by which the cardiovascular risk factor oxLDL is involved in Alzheimer's disease.

Apolipoprotein E3 (ApoE3) but not ApoE4 protects against synaptic loss through increased expression of protein kinase C epsilon

Synaptic loss is the earliest pathological change in Alzheimer disease (AD) and is the pathological change most directly correlated with the degree of dementia. ApoE4 is the major genetic risk factor for the age-dependent form of AD, which accounts for 95% of cases. Here we show that in synaptic networks formed from primary hippocampal neurons in culture, apoE3, but not apoE4, prevents the loss of synaptic networks produced by amyloid β oligomers (amylospheroids). Specific activators of PKCε, such as 8-(2-(2-pentyl-cyclopropylmethyl)-cyclopropyl)-octanoic acid methyl ester and bryostatin 1, protected against synaptic loss by amylospheroids, whereas PKCε inhibitors blocked this synaptic protection and also blocked the protection by apoE3. Blocking LRP1, an apoE receptor on the neuronal membrane, also blocked the protection by apoE. ApoE3, but not apoE4, induced the synthesis of PKCε mRNA and expression of the PKCε protein. Amyloid β specifically blocked the expression of PKCε but had no effect on other isoforms. These results suggest that protection against synaptic loss by apoE is mediated by a novel intracellular PKCε pathway. This apoE pathway may account for much of the protective effect of apoE and reduced risk for the age-dependent form of AD. This finding supports the potential efficacy of newly developed therapeutics for AD.

Activation of protein kinase C isozymes for the treatment of dementias

Memories are much more easily impaired than improved. Dementias, a lasting impairment of memory function, occur in a variety of cognitive disorders and become more clinically dominant as the population ages. Protein kinase C is one of the "cognitive kinases," and plays an essential role in both memory acquisition and maintenance. Deficits in protein kinase C (PKC) signal cascades in neurons represent one of the earliest changes in the brains of patients with Alzheimer's disease (AD) and other types of memory impairment, including those related to cerebral ischemia and ischemic stroke. Inhibition or impairment of PKC activity results in compromised learning and memory, whereas an appropriate activation of certain PKC isozymes leads to an enhancement of learning and memory and/or antidementic effects. In preclinical studies, PKC activators have been shown to increase the expression and activity of PKC isozymes, thereby restoring PKC signaling and downstream activity, including stimulation of neurotrophic activity, synaptic/structural remodeling, and synaptogenesis in the hippocampus and related cortical areas. PKC activators also reduce the accumulation of neurotoxic amyloid and tau protein hyperphosphorylation and support anti-apoptotic processes in the brain. These observations strongly suggest that PKC pharmacology may represent an attractive area for the development of effective cognition-enhancing therapeutics for the treatment of dementias.

Loss of muscarinic M1 receptor exacerbates Alzheimer's disease-like pathology and cognitive decline

Alzheimer's disease (AD) is pathologically characterized by tau-laden neurofibrillary tangles and β-amyloid deposits. Dysregulation of cholinergic neurotransmission has been implicated in AD pathogenesis, contributing to the associated memory impairments; yet, the exact mechanisms remain to be defined. Activating the muscarinic acetylcholine M(1) receptors (M(1)Rs) reduces AD-like pathological features and enhances cognition in AD transgenic models. To elucidate the molecular mechanisms by which M(1)Rs affect AD pathophysiological features, we crossed the 3xTgAD and transgenic mice expressing human Swedish, Dutch, and Iowa triple-mutant amyloid precursor protein (Tg-SwDI), two widely used animal models, with the M(1)R(-/-) mice. Our data show that M(1)R deletion in the 3xTgAD and Tg-SwDI mice exacerbates the cognitive impairment through mechanisms dependent on the transcriptional dysregulation of genes required for memory and through acceleration of AD-related synaptotoxicity. Ablating the M(1)R increased plaque and tangle levels in the brains of 3xTgAD mice and elevated cerebrovascular deposition of fibrillar Aβ in Tg-SwDI mice. Notably, tau hyperphosphorylation and potentiation of amyloidogenic processing in the mice with AD lacking M(1)R were attributed to changes in the glycogen synthase kinase 3β and protein kinase C activities. Finally, deleting the M(1)R increased the astrocytic and microglial response associated with Aβ plaques. Our data highlight the significant role that disrupting the M(1)R plays in exacerbating AD-related cognitive decline and pathological features and provide critical preclinical evidence to justify further development and evaluation of selective M(1)R agonists for treating AD.

Protein kinase C-regulated aβ production and clearance

Alzheimer's disease (AD) is the most common form of dementia among the elderly population. AD, which is characterized as a disease of cognitive deficits, is mainly associated with an increase of amyloid β-peptide (Aβ) in the brain. A growing body of recent studies suggests that protein kinase C (PKC) promotes the production of the secretory form of amyloid precursor protein (sAPPα) via the activation of α-secretase activity, which reduces the accumulation of pathogenic Aβ levels in the brain. Moreover, activation of PKCα and mitogen-activated protein kinase (MAPK) is known to increase sAPPα. A novel type of PKC, PKCε, activates the Aβ degrading activity of endothelin converting enzyme type 1 (ECE-1), which might be mediated via the MAPK pathway as well. Furthermore, dysregulation of PKC-MAPK signaling is known to increase Aβ levels in the brain, which results in AD phenotypes. Here, we discuss roles of PKC in Aβ production and clearance and its implication in AD.

L-3-n-butylphthalide improves cognitive impairment and reduces amyloid-beta in a transgenic model of Alzheimer's disease

Alzheimer's disease (AD) is an age-related, progressive neurodegenerative disorder that occurs gradually and results in memory, behavior, and personality changes. L-3-n-butylphthalide (L-NBP), an extract from seeds of Apium graveolens Linn (Chinese celery), has been demonstrated to have neuroprotective effects on ischemic, vascular dementia, and amyloid-beta (Abeta)-infused animal models. In the current study, we examined the effects of L-NBP on learning and memory in a triple-transgenic AD mouse model (3xTg-AD) that develops both plaques and tangles with aging, as well as cognitive deficits. Ten-month-old 3xTg-AD mice were given 15 mg/kg L-NBP by oral gavage for 18 weeks. L-NBP treatment significantly improved learning deficits, as well as long-term spatial memory, compared with vehicle control treatment. L-NBP treatment significantly reduced total cerebral Abeta plaque deposition and lowered Abeta levels in brain homogenates but had no effect on fibrillar Abeta plaques, suggesting preferential removal of diffuse Abeta deposits. Furthermore, we found that L-NBP markedly enhanced soluble amyloid precursor protein secretion (alphaAPPs), alpha-secretase, and PKCalpha expression but had no effect on steady-state full-length APP. Thus, L-NBP may direct APP processing toward a non-amyloidogenic pathway and preclude Abeta formation in the 3xTg-AD mice. The effect of l-NBP on regulating APP processing was further confirmed in neuroblastoma SK-N-SH cells overexpressing wild-type human APP(695) (SK-N-SH APPwt). L-NBP treatment in 3xTg-AD mice also reduced glial activation and oxidative stress compared with control treatment. L-NBP shows promising preclinical potential as a multitarget drug for the prevention and/or treatment of Alzheimer's disease.

Protein kinase C activators as synaptogenic and memory therapeutics

The last decade has witnessed a rapid progress in understanding of the molecular cascades that may underlie memory and memory disorders. Among the critical players, activity of protein kinase C (PKC) isoforms is essential for many types of learning and memory and their dysfunction, and is critical in memory disorders. PKC inhibition and functional deficits lead to an impairment of various types of learning and memory, consistent with the observations that neurotoxic amyloid inhibits PKC activity and that transgenic animal models with PKCbeta deficit exhibit impaired capacity in cognition. In addition, PKC isozymes play a regulatory role in amyloid production and accumulation. Restoration of the impaired PKC signal pathway pharmacologically results in an enhanced memory capacity and synaptic remodeling / repair and synaptogenesis, and, therefore, represents a potentially important strategy for the treatment of memory disorders, including Alzheimer's dementia. The PKC activators, especially those that are isozyme-specific, are a new class of drug candidates that may be developed as future memory therapeutics.

Disease-modifying approach to the treatment of Alzheimer's disease: from alpha-secretase activators to gamma-secretase inhibitors and modulators

In the last decade, advances in understanding the neurobiology of Alzheimer's disease (AD) have translated into an increase in clinical trials assessing various potential AD treatments. At present, drugs used for the treatment of AD only slightly delay the inevitable symptomatic progression of the disease and do not affect the main neuropathological hallmarks of the disease, i.e. senile plaques and neurofibrillary tangles. Brain accumulation of oligomeric species of beta-amyloid (A beta) peptides, the principal components of senile plaques, is believed to play a crucial role in the development of AD. Based on this hypothesis, huge efforts are being made to identify drugs able to interfere with proteases regulating A beta formation from amyloid precursor protein (APP). Compounds that stimulate alpha-secretase, the enzyme responsible for non-amyloidogenic metabolism of APP, are being developed and one of these, EHT-0202, has recently commenced evaluation in a phase II study. The discovery of inhibitors of beta-secretase (memapsin-2, beta-amyloid cleaving enzyme-1 [BACE-1]), the enzyme that regulates the first step of amyloidogenic APP metabolism, has proved to be particularly difficult because of inherent medicinal chemistry issues and only one compound (CTS-21166) has proceeded to clinical testing. Conversely, several compounds that inhibit gamma-secretase, the pivotal enzyme that generates A beta, have been identified, the most advanced being LY-450139 (semagacestat), presently in phase III clinical development. There has been considerable disappointment over the failure of a phase III study of tarenflurbil, a compound believed to modulate the activity of gamma-secretase, after encouraging phase II findings. Nevertheless, other promising gamma-secretase modulators are being developed and are approaching clinical testing. All these therapeutic approaches increase the hope of slowing the rate of decline in patients with AD and modifying the natural history of this devastating disease within the next 5 years.

Beyond the neurotransmitter-focused approach in treating Alzheimer's disease: drugs targeting beta-amyloid and tau protein

Drugs currently used to treat Alzheimer's Disease (AD) have limited therapeutic value and do not affect the main neuropathological hallmarks of the disease, i.e., senile plaques and neurofibrillar tangles. Senile plaques are mainly formed of beta-amyloid (Abeta), a 42-aminoacid peptide. Neurofibrillar tangles are composed of paired helical filaments of hyperphosphorylated tau protein. New, potentially disease-modifying, therapeutic approaches are targeting Abeta and tau protein. Drugs directed against Abeta include active and passive immunization, that have been found to accelerate Abeta clearance from the brain. The most developmentally advanced monoclonal antibody directly targeting Abeta is bapineuzumab, now being studied in a large Phase III clinical trial. Compounds that interfere with proteases regulating Abeta formation from amyloid precursor protein (APP) are also actively pursued. The discovery of inhibitors of beta-secretase, the enzyme that regulates the first step of the amyloidogenic metabolism of APP, has been revealed to be particularly difficult due to inherent medicinal chemistry problems, and only one compound (CTS-21166) has reached clinical testing. Conversely, several compounds that inhibit gamma-secretase, the pivotal enzyme that generates Abeta, have been identified, the most advanced being LY-450139 (semagacestat), now in Phase III clinical development. Compounds that stimulate alpha-secretase, the enzyme responsible for the non-amyloidogenic metabolism of APP, are also being developed, and one of them, EHT-0202, has recently entered Phase II testing. Potent inhibitors of Abeta aggregation have also been identified, and one of such compounds, PBT-2, has provided encouraging neuropsychological results in a recently completed Phase II study. Therapeutic approaches directed against tau protein include inhibitors of glycogen synthase kinase- 3 (GSK-3), the enzyme responsible for tau phosphorylation and tau protein aggregation inhibitors. NP-12, a promising GSK-3 inhibitor, is being tested in a Phase II study, and methylthioninium chloride, a tau protein aggregation inhibitor, has given initial encouraging results in a 50-week study. With all these approaches on their way, the hope for disease-modifying therapy in this devastating disease may become a reality in the next 5 years.

Phorbol-12-myristate 13-acetate acting through protein kinase Cepsilon induces translocator protein (18-kDa) TSPO gene expression

Translocator protein (TSPO) is an 18-kDa cholesterol-binding protein that is expressed at high levels in steroid synthesizing and several cancer cells where it is involved in steroidogenesis and cell proliferation, respectively. The factors regulating Tspo expression are unknown. We analyzed Tspo transcriptional responses to the tumor promoter, phorbol-12-myristate 13-acetate (PMA), in cells with varying TSPO levels. PMA induced Tspo promoter activity and Tspo mRNA levels in TSPO-poor nonsteroidogenic cells (NIH-3T3 fibroblasts and COS-7 kidney) but not in TSPO-rich steroidogenic cells (MA-10 Leydig) with high basal Tspo transcriptional activity. The stimulatory effect of PMA was mediated by an 805-515-bp region upstream of the transcription start site. Electrophoretic mobility shift assay (EMSA) revealed that PMA induced binding of c-jun and GA-binding protein transcription factor (GABP-alpha) to their respective activator protein 1 (AP1) and v-ets erythroblastosis virus E26 oncogene homologue (Ets) sites in this region. Protein kinase C (PKC)-specific inhibitors blocked PMA induction of Tspo promoter activity with an inhibition profile suggestive of involvement of PKCepsilon. PKCepsilon expression correlated with TSPO content in the three cell lines. In NIH-3T3 cells, PKCepsilon overexpression induced Tspo promoter activity and mRNA levels and enhanced PMA-induced up regulation of c-jun and TSPO. In MA-10 cells, a PKCepsilon-specific translocation inhibitor peptide reduced basal Tspo promoter activity. PKCepsilon siRNA pool reduced PKCepsilon and TSPO levels in MA-10 cells indicating a role for PKCepsilon in regulating TSPO expression. Taken together, these data suggest that elevated TSPO expression in steroidogenic cells may be due to high constitutive expression of PKCepsilon that renders them unresponsive to further induction while PMA activation of PKCepsilon drives inducible TSPO expression in nonsteroidogenic cells, likely through AP1 and Ets.

Enhanced generation of Alzheimer's amyloid-beta following chronic exposure to phorbol ester correlates with differential effects on alpha and epsilon isozymes of protein kinase C

Alzheimer's amyloid precursor protein (APP) sorting and processing are modulated through signal transduction mechanisms regulated by protein phosphorylation. Notably, protein kinase C (PKC) appears to be an important component in signaling pathways that control APP metabolism. PKCs exist in at least 11 conventional and unconventional isoforms, and PKCalpha and PKCepsilon isoforms have been specifically implicated in controlling the generation of soluble APP and amyloid-beta (Abeta) fragments of APP, although identification of the PKC substrate phospho-state-sensitive effector proteins remains challenging. In the current study, we present evidence that chronic application of phorbol esters to cultured cells in serum-free medium is associated with several phenomena, namely: (i) PKCalpha down-regulation; (ii) PKCepsilon up-regulation; (iii) accumulation of APP and/or APP carboxyl-terminal fragments in the trans Golgi network; (iv) disappearance of fluorescence from cytoplasmic vesicles bearing a green fluorescent protein tagged form of APP; (v) insensitivity of soluble APP release following acute additional phorbol application; and (vi) elevated cellular APP mRNA levels and holoprotein, and secreted Abeta. These data indicate that, unlike acute phorbol ester application, which is accompanied by lowered Abeta generation, chronic phorbol ester treatment causes differential regulation of PKC isozymes and increased Abeta generation. These data have implications for the design of amyloid-lowering strategies based on modulating PKC activity.

Metabolism of amyloid precursor protein in COS cells transfected with a beta-secretase candidate

Thimet oligopeptidase (TOP) is a thiol- andmetallo-dependent peptidase and has been shown to beone of the beta-secretase candidates. TOPexpressed in COS cells cleaved amyloid precursorprotein (APP) at the beta-secretase site, and wefound a proteolytic product of APP called secretedform of APP by beta-secretase (sAPPbeta) in theconditioned media. Here we demonstrate thatsAPPbeta was increased in conditioned media whenTOP was coexpressed in COS cells with APP and treatedwith an ADAM inhibitor SI-27. In addition, althoughTOP expressed in COS cell was localized at nuclei orGolgi apparatus, it exclusively colocalized at Golgiapparatus when APP was coexpressed with TOP.

Isoform-specific contribution of protein kinase C to prion processing

The cellular prion protein (PrP(c)) undergoes a physiological cleavage between amino acids 111 and 112, thereby leading to the secretion of an amino-terminal fragment referred to as N1. This proteolytic event is either constitutive or regulated by protein kinase C (PKC) and is operated by the disintegrins ADAM9/ADAM10 or ADAM17 respectively. We recently showed that the stimulation of the M1/M3 muscarinic receptors potentiates this cleavage via the phosphorylation and activation of ADAM17. We have examined the contribution of various PKC isoforms in the regulated processing of PrP(c). First we show that the PDBu- and carbachol-stimulated N1 secretions are blocked by the general PKC inhibitor GF109203X. We establish that HEK293 and human-derived rhabdhomyosarcoma cells over-expressing constitutively active PKCalpha, PKCdelta or PKCepsilon, but not PKCzeta, produce increased amounts of N1 and harbor enhanced ability to hydrolyze the fluorimetric substrate of ADAM17, JMV2770. Conversely, over-expression of the corresponding dominant negative proteins abolishes PDBU-stimulated N1 secretion and restores N1 to levels comparable to constitutive production. Moreover, deletion of PKCalpha lowers N1 recovery in primary cultured fibroblasts. Importantly, mutation of threonine 735 of ADAM17 significantly lowers the PDBu-induced N1 formation while transient over-expression of constitutively active PKCalpha, PKCdelta or PKCepsilon, but not PKCzeta, induced both the phosphorylation of ADAM17 on its threonine residues and N1 secretion. As a corollary, T735A mutation concomitantly reversed PKCalpha-, PKCdelta- and PKCepsilon-induced ADAM17 phosphorylation and N1 recovery. Finally, we established that PKCepsilon-dependent N1 production is fully prevented by ADAM17 deficiency. Altogether, the present results provide strong evidence that the activation of PKCalpha, delta and epsilon, but not zeta, isoforms leads to increased N1 secretion via the phosphorylation and activation of ADAM17, a process that likely accounts for M1/M3 muscarinic receptors-mediated control of N1 production.

Overexpression of amyloid precursor protein reduces epsilon protein kinase C levels

Alzheimer's disease (AD) is characterized by extracellular deposits of amyloid beta peptide (Abeta), a peptide that is generated upon proteolytic cleavage of amyloid precursor protein (APP). The events leading to the development of AD and their sequence are not yet fully understood. Protein kinase C (PKC) has been suggested to have a significant role in controlling neuronal degeneration and in the aberrant signal transduction taking place in AD. Several studies document a deficit in PKC levels and activity in brains of AD patients when compared with those of normal controls. Such a decrease in PKC could have serious implications since certain PKC isozymes were shown to drive the APP proteolytic cleavage into a non-amyloidogenic pathway. Reduced levels of distinct PKC isozymes could thus contribute to driving APP processing toward an amyloidogenic pathway. The direct cause for the down-regulation of PKC in AD brains is still unknown. In that respect, we tested in this study whether APP may play a role in PKC reduction. We show in three different cell lines (CHO, COS and BOSC) that overexpression of APP leads to decreased PKC levels. This decrease was found to be specific for the epsilon PKC isozyme whereas the levels of delta, alpha and conventional PKC remained unchanged. Furthermore, we observed this decrease for both active, membrane-associated and inactive, cytosolic epsilon PKC. APP-driven decrease in epsilon PKC is most likely mediated by a factor in the culture medium, since transfer of medium from cultured cells overexpressing APP to naïve, non-overexpressing cells, has also led to the selective decrease in epsilon PKC levels. Taken together, our results suggest that APP expression levels may play a role in the decrease of epsilon PKC levels in AD brains and could thus affect the responsiveness of AD brain tissues to growth factors and neurotransmitters.

Effects of huperzine A on amyloid precursor protein processing and beta-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.

Bryostatin-1: pharmacology and therapeutic potential as a CNS drug

Bryostatin-1 is a powerful protein kinase C (PKC) agonist, activating PKC isozymes at nanomolar concentrations. Pharmacological studies of bryostatin-1 have mainly been focused on its action in preventing tumor growth. Emerging evidence suggests, however, that bryostatin-1 exhibits additional important pharmacological activities. In preclinical studies bryostatin-1 has been shown at appropriate doses to have cognitive restorative and antidepressant effects. The underlying pharmacological mechanisms may involve an activation of PKC isozymes, induction of synthesis of proteins required for long-term memory, restoration of stress-evoked inhibition of PKC activity, and reduction of neurotoxic amyloid accumulation and tau protein hyperphosphorylation. The therapeutic potential of bryostatin-1 as a CNS drug should be further explored.

PKCepsilon increases endothelin converting enzyme activity and reduces amyloid plaque pathology in transgenic mice

Deposition of plaques containing amyloid beta (Abeta) peptides is a neuropathological hallmark of Alzheimer's disease (AD). Here we demonstrate that neuronal overexpression of the epsilon isozyme of PKC decreases Abeta levels, plaque burden, and plaque-associated neuritic dystrophy and reactive astrocytosis in transgenic mice expressing familial AD-mutant forms of the human amyloid precursor protein (APP). Compared with APP singly transgenic mice, APP/PKCepsilon doubly transgenic mice had decreased Abeta levels but showed no evidence for altered cleavage of APP. Instead, PKCepsilon overexpression selectively increased the activity of endothelin-converting enzyme, which degrades Abeta. The activities of other Abeta-degrading enzymes, insulin degrading enzyme and neprilysin, were unchanged. These results indicate that increased neuronal PKCepsilon activity can promote Abeta clearance and reduce AD neuropathology through increased endothelin-converting enzyme activity.

Synthesis and Pharmacological Evaluation of 8- and 9-Substituted Benzolactam-V8 Derivatives as Potent Ligands for Protein Kinase C, a Therapeutic Target for Alzheimer's Disease

A central element in the pathophysiology of Alzheimer's disease (AD) is the formation of amyloid plaques, which result from abnormal processing of the amyloid precursor protein (APP). The processing of APP is largely provided by three key enzymes, namely the alpha-, beta-, and gamma-secretases. As the latter two contribute to the formation of neurotoxic Abeta fragments while alpha-secretase does not, a decrease in the amyloidogenic products can be brought about either by inhibition of the beta- and gamma-secretases or through the activation of alpha-secretase. It is now known that the activation of protein kinase C (PKC) enhances alpha-secretase activity and therefore represents a possible target for the development of agents urgently needed for the treatment of this devastating neurodegenerative disorder. In the present study, new benzolactam-V8-based PKC activators were synthesized and tested for their binding affinity toward PKCalpha. All compounds tested showed binding values in the nanomolar concentration range. In accordance with previous publications, 9-substitution dramatically increased PKC binding affinity in comparison with the corresponding 8-substituted analogues. In addition to the location of the side chain on the aromatic ring, the binding affinities of these benzolactams were found to depend on the orientation, length, and electronic properties of this appendage. An interesting decrease in binding affinity was found for the 9-thienyl analogue 13, suggesting adverse electronic interactions of the sulfur atom with PKC or parts of the cellular membrane.

Phospholipase A2 is involved in muscarinic receptor-mediated sAPPalpha release independently of cyclooxygenase or lypoxygenase activity in SH-SY5Y cells

The release of soluble amyloid precursor protein alpha (sAPPalpha), produced during alpha-secretase processing by cleavage within the beta amyloid peptide domain of APP, is highly regulated by several external and internal signals. Because evidence suggests the involvement of inflammatory processes in the pathology of Alzheimer's disease and APP formation, we examined the involvement of the phospholipase A2 (PLA2) pathway and of its downstream cyclooxygenase (COX) and lipoxygenase (LOX) pathways in the regulation of sAPPalpha, release induced by muscarinic receptor activation in SH-SY5Y cells. The amount of sAPP released into the culture medium was analyzed using a monoclonal 6E10 antibody detecting sAPPalpha. Treatment with the PLA2 inhibitor, manoalide, blocked the release of oxoM (muscarinic receptor agonist)-stimulated sAPPalpha, and the muscarinic receptor-mediated sAPPalpha release was increased by the non-selective PLA2 activator mellitin. COX and LOX inhibitors inhibited exogenous AA-induced sAPPalpha release, but upregulated basal constitutive sAPPalpha release. However, treatment with COX or LOX inhibitors failed to significantly change oxoM-stimulated sAPPalpha release, and furthermore, muscarinic receptor activation inhibited AA-stimulated COX activity. Our results suggest that sAPPalpha release induced by muscarinic receptor activation is regulated by AA generation via PLA2 activation independently of COX and LOX activities, but that the COX and LOX pathways are possibly involved in the constitutive release of sAPPalpha.

Mechanism of neuroprotective action of the anti-Parkinson drug rasagiline and its derivatives

The mitochondria are directly involved in cell survival and death. Drugs that protect mitochondria viability and prevent apoptotic cascade mechanisms involved in mitochondrial permeability transition pore (MPTp) will be cytoprotective. Rasagiline (N-propargyl-1R-aminoindan) is a novel, highly potent irreversible monoamine oxidase (MAO) B inhibitor, anti-Parkinson drug. Unlike selegiline, rasagiline is not derived from amphetamine, is not metabolized to neurotoxic l-methamphetamine derivative, nor does it have sympathomimetic activity. Rasagiline is effective as monotherapy or adjunct to L-dopa for patients with early and late Parkinson's disease (PD), and adverse events do not occur with greater frequency in subjects receiving rasagiline than those on placebo. Controlled studies indicate that it might have a disease-modifying effect in PD that may be related to neuroprotection. Its S-isomer, TVP1022, is a relatively inactive MAO inhibitor. However, both drugs have similar neuroprotective activities in neuronal cell cultures in response to various neurotoxins and in vivo (global ischemia, neurotrauma, head injury, anoxia, etc.), indicating that MAO inhibition is not a pre-requisite for neuroprotection. Structure activity studies have shown that the neuroprotective activity is associated with the propargyl moiety of rasagiline which protects mitochondrial viability and MPTp by activating Bcl-2 and protein kinase C (PKC), and down regulating pro-apoptotic FAS and Bax. Rasagiline and its derivatives also process amyloid precursor protein (APP) to the neuroprotective-neurotrophic soluble APP alpha (sAPPalpha) by PKC and MAP kinase-dependent activation of alpha-secretase. The neuroprotective activity of propargylamine has led us to develop novel bifunctional neuroprotective iron-chelating MAO-inhibiting drugs possessing propargyl moiety for the treatment of other neurodegenerative diseases.

Differential involvement of protein kinase C alpha and epsilon in the regulated secretion of soluble amyloid precursor protein

We investigated the differential role of protein kinase C (PKC) isoforms in the regulated proteolytic release of soluble amyloid precursor protein (sAPPalpha) in SH-SY5Y neuroblastoma cells. We used cells stably transfected with cDNAs encoding either PKCalpha or PKCepsilon in the antisense orientation, producing a reduction of the expression of PKCalpha and PKCepsilon, respectively. Reduced expression of PKCalpha and/or PKCepsilon did not modify the response of the kinase to phorbol ester stimulation, demonstrating translocation of the respective isoforms from the cytosolic fraction to specific intracellular compartments with an interesting differential localization of PKCalpha to the plasma membrane and PKCepsilon to Golgi-like structures. Reduced expression of PKCalpha significantly impaired the secretion of sAPPalpha induced by treatment with phorbol esters. Treatment of PKCalpha-deficient cells with carbachol induced a significant release of sAPPalpha. These results suggest that the involvement of PKCalpha in carbachol-induced sAPPalpha release is negligible. The response to carbachol is instead completely blocked in PKCepsilon-deficient cells suggesting the importance of PKCepsilon in coupling cholinergic receptors with APP metabolism.

Therapeutic effects of PKC activators in Alzheimer's disease transgenic mice

Alzheimer's disease (AD) characteristically presents with early memory loss. Regulation of K(+) channels, calcium homeostasis, and protein kinase C (PKC) activation are molecular events that have been implicated during associative memory which are also altered or defective in AD. PKC is also involved in the processing of the amyloid precursor protein (APP), a central element in AD pathophysiology. In previous studies, we demonstrated that benzolactam (BL), a novel PKC activator, reversed K(+) channels defects and enhanced secretion of APP alpha in AD cells. In this study we present data showing that another PKC activator, bryostatin 1, at subnanomolar concentrations dramatically enhances the secretion of the alpha-secretase product sAPP alpha in fibroblasts from AD patients. We also show that BL significantly increased the amount of sAPP alpha and reduced A beta 40 in the brains of APP[V717I] transgenic mice. In a more recently developed AD double-transgenic mouse, bryostatin was effective in reducing both brain A beta 40 and A beta 42. In addition, bryostatin ameliorated the rate of premature death and improved behavioral outcomes. Collectively, these data corroborate PKC and its activation as a potentially important means of ameliorating AD pathophysiology and perhaps cognitive impairment, thus offering a promising target for drug development. Because bryostatin 1 is devoid of tumor-promoting activity and is undergoing numerous clinical studies for cancer treatment in humans, it might be readily tested in patients as a potential therapeutic agent for Alzheimer's disease.

Amyloid beta peptide directly inhibits PKC activation

A putative protein kinase C (PKC) pseudosubstrate domain in beta amyloid (Abeta) suggests a potential interaction between Abeta and PKC. In this study, we investigated whether and how Abeta interacts with PKC. Abeta peptides inhibited PKC phosphorylation in a dose-dependent manner in cell-free in vitro condition, suggesting a direct interaction between Abeta and PKC. Experiments involving deletion of the Abeta sequence indicated that the putative PKC pseudosubstrate domain (Abeta 28-30) is critical for Abeta-PKC interaction. Addition of Abeta peptides to cultured B103 cells reduced the activated forms of PKCalpha and PKCepsilon. It also inhibited phorbol-12,13-dibutyrate (PDBu)-induced membrane translocation of PKCalpha and PKCepsilon without altering their expression levels, indicating that activation of intracellular PKC is inhibited by treatment of Abeta peptides. These results suggest that Abeta peptides inhibit PKC activation via direct interactions, which may play a role in pathogenesis of AD.

Neuroprotection and neurorescue against Abeta toxicity and PKC-dependent release of nonamyloidogenic soluble precursor protein by green tea polyphenol (-)-epigallocatechin-3-gallate

Green tea extract and its main polyphenol constituent (-)-epigallocatechin-3-gallate (EGCG) possess potent neuroprotective activity in cell culture and mice model of Parkinson's disease. The central hypothesis guiding this study is that EGCG may play an important role in amyloid precursor protein (APP) secretion and protection against toxicity induced by beta-amyloid (Abeta). The present study shows that EGCG enhances (approximately 6-fold) the release of the non-amyloidogenic soluble form of the amyloid precursor protein (sAPPalpha) into the conditioned media of human SH-SY5Y neuroblastoma and rat pheochromocytoma PC12 cells. sAPPalpha release was blocked by the hydroxamic acid-based metalloprotease inhibitor Ro31-9790, which indicated mediation via alpha-secretase activity. Inhibition of protein kinase C (PKC) with the inhibitor GF109203X, or by down-regulation of PKC, blocked the EGCG-induced sAPPalpha secretion, suggesting the involvement of PKC. Indeed, EGCG induced the phosphorylation of PKC, thus identifying a novel PKC-dependent mechanism of EGCG action by activation of the non-amyloidogenic pathway. EGCG is not only able to protect, but it can rescue PC12 cells against the beta-amyloid (Abeta) toxicity in a dose-dependent manner. In addition, administration of EGCG (2 mg/kg) to mice for 7 or 14 days significantly decreased membrane-bound holoprotein APP levels, with a concomitant increase in sAPPalpha levels in the hippocampus. Consistently, EGCG markedly increased PKCalpha and PKC in the membrane and the cytosolic fractions of mice hippocampus. Thus, EGCG has protective effects against Abeta-induced neurotoxicity and regulates secretory processing of non-amyloidogenic APP via PKC pathway.

Role of protein kinase Calpha in the regulated secretion of the amyloid precursor protein

Protein kinase C (PKC) has a key role in the signal transduction machinery involved in the regulation of amyloid precursor protein (APP) metabolism. Direct and indirect receptor-mediated activation of PKC has been shown to increase the release of soluble APP (sAPPalpha) and reduce the secretion of beta-amyloid peptides. Experimental evidence suggests that specific isoforms of PKC, such as PKCalpha and PKC epsilon, are involved in the regulation of APP metabolism. In this study, we characterized the role of PKCalpha in the regulated secretion of APP using wild-type SH-SY5Y neuroblastoma cells and cells transfected with a plasmid expressing PKCalpha antisense cDNA. Cells expressing antisense PKCalpha secrete less sAPPalpha in response to phorbol esters. In contrast, carbachol increases the secretion of sAPPalpha to similar levels in wild-type cells and in cells transfected with antisense PKCalpha by acting on APP metabolism through an indirect pathway partially involving the activation of PKC. These results suggest that the direct PKC-dependent activation of the APP secretory pathway is compromised by reduced PKCalpha expression and a specific role of this isoform in these mechanisms. On the other hand, indirect pathways that are also partially dependent on the mitogen-activated protein kinase signal transduction mechanism remain unaffected and constitute a redundant, compensatory mechanism within the APP secretory pathway.

A secreted form of human ADAM9 has an alpha-secretase activity for APP

ADAM9 (MDC9, meltrin gamma) is a member of the ADAM family of metalloproteases, which play important roles in cell-cell fusion, intracellular signaling, and other cellular functions. Here we cloned a novel form of human ADAM9, designated hADAM9s (s for short), which lacks the carboxyl-terminus. Human ADAM9s was found to be secreted from transfected COS cells. RT-PCR analysis demonstrated that the mRNA for hADAM9s is expressed in human brain, liver, heart, kidney, lung, and trachea. When hADAM9s was co-expressed in COS cells with APP and treated with phorbol ester, the APP was digested exclusively at the alpha-secretory site. These results suggest that hADAM9s has an alpha-secretase-like activity for APP. Non-amyloidgenic cleavage of APP may occur at the plasma membrane. Our new results support a new therapeutic strategy to decrease in the Abeta content by directly activating ADAM9 in the extracellular space.

A role for MAP kinase in regulating ectodomain shedding of APLP2 in corneal epithelial cells

We previously reported an increased secretion of amyloid precursor-like protein 2 (APLP2) in the healing corneal epithelium. The present study sought to investigate signal transduction pathways involved in APLP2 shedding in vitro. APLP2 was constitutively shed and released into culture medium in SV40-immortalized human corneal epithelial cells as assessed by Western blotting, flow cytometry, and indirect immunofluorescence. Activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) caused significant increases in APLP2 shedding. This was inhibited by staurosporine and a PKC-epsilon-specific, N-myristoylated peptide inhibitor. Epidermal growth factor (EGF) also induced APLP2 accumulation in culture medium. Basal APLP2 shedding as well as that induced by PMA and EGF was blocked by a mitogen-activated protein kinase (MAPK) kinase inhibitor, U-0126. Our results suggest that MAPK activity accounts for basal as well as PKC- and EGF-induced APLP2 shedding. In addition, PKC-epsilon may be involved in the induction of APLP2 shedding in corneal epithelial cells.

Protein kinase C epsilon suppresses Abeta production and promotes activation of alpha-secretase

Deposition of plaques containing Abeta is considered important in the pathogenesis of Alzheimer's disease. Phorbol esters that activate protein kinase C (PKC) promote alpha-secretase-mediated processing of the beta amyloid precursor protein (APP), which generally reduces formation of Abeta. To determine which PKC isozymes mediate this process, we studied CHO cells that express human APP751. Phorbol 12-myristate, 13-acetate (PMA)-stimulated APP secretion, which was reduced by a general PKC inhibitor bisindoylmaleimide I, but not by Gö 6976, which inhibits PKCalpha, beta, gamma, and mu. Since PKCdelta and epsilon were the only other PMA-sensitive isozymes present, we studied cells that express selective peptide inhibitors of these isozymes. Expression of the PKCepsilon inhibitor inhibited PMA-induced APPs secretion and suppression of Abeta production. In contrast, the PKCdelta inhibitor had no effect. These results provide evidence that PKCepsilon decreases Abeta production by promoting alpha-secretase mediated cleavage of APP.

Secretases as therapeutic targets for the treatment of Alzheimer's disease

The extracellular deposition of short amyloid peptides in the brain of patients is thought to be a central event in the pathogenesis of Alzheimer's Disease. The generation of the amyloid peptide occurs via a regulated cascade of cleavage events in its precursor protein, A beta PP. At least three enzymes are responsible for A beta PP proteolysis and have been tentatively named alpha-, beta- and gamma-secretases. The recent identification of several of these secretases is a major leap in the understanding how these secretases regulate amyloid peptide formation. Members of the ADAM family of metalloproteases are involved in the non-amyloidogenic alpha-secretase pathway. The amyloidogenic counterpart pathway is initiated by the recently cloned novel aspartate protease named BACE. The available data are conclusive and crown BACE as the long-sought beta-secretase. This enzyme is a prime candidate drug target for the development of therapy aiming to lower the amyloid burden in the disease. Finally, the gamma-secretases are intimately linked to the function of the presenilins. These multi-transmembrane domain proteins remain intriguing study objects. The hypothesis that the presenilins constitute a complete novel type of protease family, and are cleaving A beta PP within the transmembrane region, remains an issue of debate. Several questions remain unanswered and direct proof that they exert catalytic activity is still lacking. The subcellular localization of presenilins in neurons, their integration in functional multiprotein complexes and the recent identification of additional modulators of gamma-secretase, like nicastrin, indicate already that several players are involved. Nevertheless, the rapidly increasing knowledge in this area is already paving the road towards selective inhibitors of this secretase as well. It is hoped that such drugs, possibly in concert with the experimental vaccination therapies that are currently tested, will lead to a cure of this inexorable disease.

Effects of apolipoprotein E (apoE) isoforms, beta-amyloid (Abeta) and apoE/Abeta complexes on protein kinase C-alpha (PKC-alpha) translocation and amyloid precursor protein (APP) processing in human SH-SY5Y neuroblastoma cells and fibroblasts

We investigated the effects of different apolipoprotein E (apoE) isoforms, Abeta (1-42), and apoE/Abeta complexes on PKC-alpha translocation and APP processing in human SH-SY5Y neuroblastoma cells and fibroblasts. Treatment of cells with either 10 nM apoE3 or apoE4, 10 microM Abeta (1-42), or apoE/Abeta complexes induced significant translocation of PKC-alpha in both cell types. Effects were seen using both human recombinant apoE and apoE loaded into beta-very low density lipoprotein (beta-VLDL) particles. Time course (5-24 h) studies of APP processing revealed that some conditions induced transient or moderate increases in the secretion of proteins detected by 22C11. In contrast, the secretion of alpha-secretase cleaved APP was either not modified or transiently decreased, as determined by immunoblotting with the antibody 6E10. These results suggest that apoE, Abeta (1-42) and apoE/Abeta complexes can modulate PKC activity but do not have major consequences for APP processing. These effects could contribute to the reported PKC alterations seen in AD. However, it is unlikely that the contribution of different apoE isoforms to AD pathology occurs via effects on APP processing.

Blockade of PKC epsilon activation attenuates phorbol ester-induced increase of alpha-secretase-derived secreted form of amyloid precursor protein

The role of PKC epsilon in amyloid precursor protein (APP) processing was investigated using APP-overexpressing B103 cells. As reported previously, a PKC activator, phorbol-12,13-dibutyrate (PDBu), enhanced secretion of APP alpha, and this effect was blocked by a PKC inhibitor, GF109203X in this system. Selective inhibition of PKC epsilon by overexpressing the PKC epsilon V1 region, which binds specifically to the receptor for activated C-kinase (RACK), blocked PDBu-induced enhancement of APP alpha secretion as well as PDBu-induced decrease in beta-secretase-derived APP C-terminal fragment production. On the other hand, the level of PKC epsilon, but not that of PKC alpha or PKC gamma, was substantially lower in the brains of Alzheimer's disease patients compared to age-matched controls. These results add to a growing body of evidence that PKC epsilon plays an important role in modulating APP processing, and suggest that reduced PKC epsilon activity may contribute to the development of Alzheimer's disease.

Nicergoline stimulates protein kinase C mediated alpha-secretase processing of the amyloid precursor protein in cultured human neuroblastoma SH-SY5Y cells

We investigated the ability of the antidementia agents, nicergoline, aniracetam and hydergine to stimulate PKC mediated alpha-secretase amyloid precursor protein (APP) processing in cultured human neuroblastoma SH-SY5Y cells. Western immunoblotting of cell conditioned media using the Mabs 22C11 and 6E10 revealed the presence of 2 bands with molecular mass of 90 and 120 kDa, corresponding to possible alternatively glycosylated forms of secreted APP (APPs). Short-term (30 min and 2 h) treatment of cells with nicergoline gave an increased intensity of both bands, compared to non-treated cells. Maximal nicergoline effects, of the order of 150-200% over basal APPs release, were seen at concentrations between 1 and 10 microM. Under the same condition, 1 microM PdBu, used as a positive control, gave 500-1000% increases of basal APPs release. In contrast, aniracetam and hydergine, did not show any effect on APPs secretion. 2 h treatment with nicergoline had no effect on cellular full-length APP levels, as determined by immunoblotting of cell extracts with 22C11 and CT15 antibodies. Immunoblotting with PKC isoform specific antibodies of soluble and membrane fractions prepared from 2 h treated cells, showed that nicergoline (50 microM) and PdBu (1 microM) both induced translocation of PKC alpha, gamma and epsilon, but not PKC beta. The involvement of PKC in mediating nicergoline stimulated APPs release was also studied using specific inhibitors. 1 microM calphostin C, a broad range PKC inhibitor, significantly reduced both PdBu (1 microM) and nicergoline (10 microM) induced APPs release. In contrast, Go6976 (1 microM), a selective PKC alpha and beta1 inhibitor, as well as the cAMP-dependent protein kinase inhibitor, H89 (1 microM) were without effect. These results indicate that nicergoline can modulate alpha-secretase APP processing by a PKC dependent mechanism that is likely to involve the gamma and epsilon isoforms of this enzyme.

Beta-amyloid precursor protein is detectable on monocytes and is increased in Alzheimer's disease

Using the anti-beta-amyloid precursor protein (betaAPP) monoclonal antibodies 4G8, 6E10 and 22C11 and flow cytometry, we report that human circulating peripheral blood monocytes display surface immunoreactivity for betaAPP. In contrast, circulating lymphocytes do not possess cell surface betaAPP immunoreactivity, despite similar levels of betaAPP expression. Immunoblotting analysis showed that monocytes, but not lymphocytes, possess an 82 kDa C-terminal betaAPP fragment consistent with a processed transmembrane species. Monocyte surface betaAPP was upregulated approximately threefold by activation with lipopolysaccharide and interferon-gamma, activation did not produce detectable betaAPP on the cell surface of lymphocytes. Surface betaAPP immunoreactivity was reduced in a normal aged population compared to normal young controls (Young = 81.07 +/- 13.67 mean fluorescence units, Aged = 36.74 +/- 3.81, p < 0.01), but was significantly increased in AD subjects compared to age-matched healthy controls (AD = 60.31 +/- 7.42, p < 0.05). Our data suggest that a proportion of peripheral A beta may be derived from monocyte/macrophages, and that defects in brain cell processing of betaAPP in AD may be shared by this readily accessible peripheral cell.

Regulation of amyloid precursor protein cleavage

Multiple lines of evidence suggest that increased production and/or deposition of the beta-amyloid peptide, derived from the amyloid precursor protein, contributes to Alzheimer's disease. A growing list of neurotransmitters, growth factors, cytokines, and hormones have been shown to regulate amyloid precursor protein processing. Although traditionally thought to be mediated by activation of protein kinase C, recent data have implicated other signaling mechanisms in the regulation of this process. Moreover, novel mechanisms of regulation involving cholesterol-, apolipoprotein E-, and stress-activated pathways have been identified. As the phenotypic changes associated with Alzheimer's disease encompass many of these signaling systems, it is relevant to determine how altered cell signaling may be contributing to increasing brain amyloid burden. We review the myriad ways in which first messengers regulate amyloid precursor protein catabolism as well as the signal transduction cascades that give rise to these effects.

Proteasome inhibitors induce the association of Alzheimer's amyloid precursor protein with Hsc73

Amyloid precursor protein (APP) is a secretory membrane-bound protein that undergoes restrictive proteolysis and degradation with a short life span in the constitutive secretory pathway or in the endosomal/lysosomal compartment. The degradation machinery, including cellular trafficking and the restrictive cleavage of APP, is poorly understood. To gain further insight into the intracellular degradation mechanism of APP, we searched for effector proteins that interact with APP. We found that a cytosolic molecular chaperon, Hsc73, effectively interacts with the cytoplasmic domain of APP in the presence of proteasome inhibitors. Hsc73 binds to the cytoplasmic domain near the post-transmembrane region of APP and not to the KFERQ-related sequence, KFFEQ, at the C-terminal tail that is assumed to be the selective targeting signal for lysosomal proteolysis. The amounts of Hsc73 that bind to several APP species such as those found in pathological Familial Alzheimer's disease (FAD), Swedish, or Dutch type mutation, are almost identical, suggesting that an abnormal conformation around the secretory cleavage site or a pathological imbalance in APP processing are not irrelevant to the efficiency of Hsc73 binding.

The secretion of soluble amyloid beta precursor protein (A beta PPs) by chick neurons in serum-free primary culture is not regulated by protein kinase C

The metabolism of amyloid precursor protein (A beta PP) in chick neurons cultured in serum-free medium is described. A beta PP immunoreactivity, detected with the 22C11 antibody, was seen at approximately 135 and approximately 120 kDa. A beta PPs (approximately 120 kDa) was released from the cells and could be detected in the culture medium without the need of a purification step. The content of A beta PPs increased with time after medium change, but was not affected by either carbachol (100 microM), glutamate (50 microM), veratrine (20 microM), oleic acid (200 microM), A23187 (5 microM), phorbol 12, 13-dibutyrate (PDBu, 1 microM), staurosporine (1 microM), Gö 6976 (1 microM) or okadaic acid (50 nM) although the combination of PDBu and okadaic acid reduced the secretion. Addition of the muscarinic receptor agonist carbachol to the neurons increased the rate of phosphoinositide breakdown. In Western blot experiments using antibodies to the alpha, beta II and epsilon isoforms of protein kinase C and conditions whereby robust signals could be seen with rat brain lysates, no immunoreactive bands that could be inhibited by appropriate positive control peptides were seen. It is concluded A beta PPs production by chick neurons in culture is mainly constitutive in nature.

Dendritic changes in Alzheimer's disease and factors that may underlie these changes

It seems likely that the Alzheimer disease (AD)-related dendritic changes addressed in this article are induced by two principally different processes. One process is linked to the plastic response associated with deafferentation, that is, long-lasting transneuronally induced regressive changes in dendritic geometry and structure. The other process is associated with severe alterations of the dendritic- and perikaryal cytoskeleton as seen in neurons with the neurofibrillary pathology of AD, that is, the formation of paired helical filaments formed by hyperphosphorylated microtubule-associated protein tau. As the development of dendritic and cytoskeletal abnormalities are at least mediated by alterations in signal transduction, this article also reviews changes in signal pathways in AD. We also discuss transgenic approaches developed to model and understand cytoskeletal abnormalities.

Developmental and aging changes in the expression of amyloid precursor protein in Down syndrome brains

We studied immunohistochemically the expression of beta-amyloid precursor protein (APP) in the frontal lobes of 18 Down syndrome (DS) patients (20 gestation weeks (GW) to 50 years) and 15 controls (17 GW to 50 years) using six purified antibodies against the secretory forms (N-terminal, N-Amy and Amy540), the Kunitz-type protease inhibitor (KPI) domain, residues 1-28 of beta protein (Affi28), and the carboxyl-terminal fragment (Ac) of APP. In the cortex of fetuses, neonates and infants, immunoreactivity for N-Amy and Ac was observed in both neurons and glial cells, and that for Affi28 in glial cells in the subpial layer in both DS patients and controls suggesting the functioning role of APP was a growth factor. This immunoreactivity disappeared in childhood and reappeared in adulthood in only DS patients. The earlier reappearance of those in DS patients from a young adult age than in normal controls may result from a gene dosage effect, since APP is encoded on chromosome 21. The N-Amy, Amy540, Affi28 and Ac immunoreactivity in glial cells in the developing white matter in the both DS patients and controls may be associated with myelination glia. Immunoreactivity for KPI was noted on the tunica media of the arteries from the neonatal period to adulthood in only DS patients. In senile plaques in DS patients, N-terminal and Affi28 immunoreactivity became detectable at the age of 32 years. N-terminal immunoreactivity in the senile plaques was noted along the periphery of the senile plaques, while that for Affi28 was around the amyloid core. Thus, each fragment of APP exhibited a different localization and time course of immunohistochemical expression. The results indicated that APP plays a role in neuronal development and that its earlier reappearance in adult DS patients is associated with the regeneration process related to aging.

Mutation of potential N-linked glycosylation sites in the Alzheimer's disease amyloid precursor protein (APP)

In order to study the mechanism of intracellular sorting and processing of the Alzheimer's disease amyloid precursor protein (APP), we deleted two potential N-linked glycosylation sites of APP by site-directed mutagenesis. Substitution of alanines for the critical asparagine residues Asn467 and Asn496 was performed. Wild-type and mutant APPs were expressed in COS-1 cells by cDNA transfection and the expressed of the protein and secretion of N-terminal large fragment was observed. The initial secretion of the mutant APP appeared to be slow compared with wild-type. In addition, we found that a distinct APP fragment, the cytosolic form, is transiently increased in the cytosol fraction of COS-1 cells. These results suggest that aberrant processing occurs following the expression of a mutant APP with Ala substituted for Asn, and that glycosylation may modulate the intracellular sorting of APP.