Protein kinase Calpha and beta1 isoforms are regulators of alpha-secretory proteolytic processing of amyloid precursor protein in vivo

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.

Autism genes and the leukocyte transcriptome in autistic toddlers relate to pathogen interactomes, infection and the immune system. A role for excess neurotrophic sAPPα and reduced antimicrobial Aβ

Prenatal and early childhood infections have been implicated in autism. Many autism susceptibility genes (206 Autworks genes) are localised in the immune system and are related to immune/infection pathways. They are enriched in the host/pathogen interactomes of 18 separate microbes (bacteria/viruses and fungi) and to the genes regulated by bacterial toxins, mycotoxins and Toll-like receptor ligands. This enrichment was also observed for misregulated genes from a microarray study of leukocytes from autistic toddlers. The upregulated genes from this leukocyte study also matched the expression profiles in response to numerous infectious agents from the Broad Institute molecular signatures database. They also matched genes related to sudden infant death syndrome and autism comorbid conditions (autoimmune disease, systemic lupus erythematosus, diabetes, epilepsy and cardiomyopathy) as well as to estrogen and thyrotropin responses and to those upregulated by different types of stressors including oxidative stress, hypoxia, endoplasmic reticulum stress, ultraviolet radiation or 2,4-dinitrofluorobenzene, a hapten used to develop allergic skin reactions in animal models. The oxidative/integrated stress response is also upregulated in the autism brain and may contribute to myelination problems. There was also a marked similarity between the expression signatures of autism and Alzheimer's disease, and 44 shared autism/Alzheimer's disease genes are almost exclusively expressed in the blood-brain barrier. However, in contrast to Alzheimer's disease, levels of the antimicrobial peptide beta-amyloid are decreased and the levels of the neurotrophic/myelinotrophic soluble APP alpha are increased in autism, together with an increased activity of α-secretase. sAPPα induces an increase in glutamatergic and a decrease in GABA-ergic synapses creating and excitatory/inhibitory imbalance that has also been observed in autism. A literature survey showed that multiple autism genes converge on APP processing and that many are able to increase sAPPalpha at the expense of beta-amyloid production. A genetically programmed tilt of this axis towards an overproduction of neurotrophic/gliotrophic sAPPalpha and underproduction of antimicrobial beta-amyloid may explain the brain overgrowth and myelination dysfunction, as well as the involvement of pathogens in autism.

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.

Analyzing the Behavior of Neuronal Pathways in Alzheimer's Disease Using Petri Net Modeling Approach

Alzheimer's Disease (AD) is the most common neuro-degenerative disorder in the elderly that leads to dementia. The hallmark of AD is senile lesions made by abnormal aggregation of amyloid beta in extracellular space of brain. One of the challenges in AD treatment is to better understand the mechanism of action of key proteins and their related pathways involved in neuronal cell death in order to identify adequate therapeutic targets. This study focuses on the phenomenon of aggregation of amyloid beta into plaques by considering the signal transduction pathways of Calpain-Calpastatin (CAST) regulation system and Amyloid Precursor Protein (APP) processing pathways along with Ca2+ channels. These pathways are modeled and analyzed individually as well as collectively through Stochastic Petri Nets for comprehensive analysis and thorough understating of AD. The model predicts that the deregulation of Calpain activity, disruption of Calcium homeostasis, inhibition of CAST and elevation of abnormal APP processing are key cytotoxic events resulting in an early AD onset and progression. Interestingly, the model also reveals that plaques accumulation start early (at the age of 40) in life but symptoms appear late. These results suggest that the process of neuro-degeneration can be slowed down or paused by slowing down the degradation rate of Calpain-CAST Complex. In the light of this study, the suggestive therapeutic strategy might be the prevention of the degradation of Calpain-CAST complexes and the inhibition of Calpain for the treatment of neurodegenerative diseases such as AD.

The thiol proteinase inhibitor E-64-d ameliorates amyloid-β-induced reduction of sAPPα secretion by reversing ceramide-induced protein kinase C down-regulation in SH-SY5Y neuroblastoma cells

In Alzheimer's disease (AD), enhancing α-secretase processing of amyloid precursor protein (APP) is an important pathway to decrease neurotoxic amyloid β (Aβ) secretion. The α-secretase is reported to be regulated by protein kinase C (PKC) and various endogenous proteins or cell surface receptors. In this report, we first examined whether Aβ reduces α-secretase activity, and showed that Aβ peptide 1-40 (0.001 and 0.01 μM) reduced the secretion of soluble amyloid precursor protein α (sAPPα) in carbachol-stimulated SH-SY5Y neuroblastoma cells. E-64-d (3 μM), which is a potent calpain inhibitor that prevents PKC degradation, ameliorated the Aβ-induced reduction of sAPPα secretion. In addition, we observed that Aβ significantly enhanced ceramide production by activating neutral sphingomyelinase. The cell-permeable ceramide analog, C2-ceramide (1 μg/mL), also reduced sAPPα secretion, and in addition, E-64-d eliminated the observed decrease of sAPPα secretion. C2-ceramide induced down-regulation of PKC-α, -β1, and -β2 isozymes in SH-SY5Y cells. These findings suggest that ceramide may play an important role in sAPPα processing by modulating PKC activity.

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.

Neuroprotective effects of new protein kinase C activator TPPB against Aβ₂₅₋₃₅ induced neurotoxicity in PC12 cells

Alzheimer's disease (AD) is pathologically characterized by presence of senile plaques in the hippocampus, which are composed mainly of extracellular deposition of a polypeptide known as the beta amyloid, the Aβ. It has been demonstrated on numerous occasions that it was the deposition and aggregation of this Aβ peptide that cause neuronal dysfunction and even finally, the dementia. Lowering the deposition of Aβ or decreasing its neurotoxicity has long been one of the purposes of AD therapy. In previous study, we reported that protein kinase C (PKC) activator TPPB could regulate APP processing by increasing α-secretase activity. In this study we further investigated the potential neuroprotective effect of TPPB against Aβ(25-35)-induced neurotoxicity in PC12 cells. The results indicated that TPPB at concentration of 1 μM could antagonize Aβ(25-35) induced cell damage as evidenced by MTT assays, LDH release and by morphological changes. Furthermore, the neuroprotection in cell viability can be blocked by inhibitors of PKC, Akt and MAPK. The experiment also indicated that TPPB could increase the phosphorylation of Akt, PKC, MARCKS and MAPK, which were inhibited by Aβ(25-35) treatment. Finally, TPPB inhibited the activation of caspase-3 induced by Aβ(25-35). Taken together, the experiment here implies that TPPB has a role against Aβ(25-35)-induced neurotoxicity in PC12 cells and may suggest its therapeutic potential in AD.

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

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

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.

Redox control of protein kinase C: cell- and disease-specific aspects

Hormones, growth factors, electrical stimulation, and cell-cell interactions regulate numerous cellular processes by altering the levels of second messengers, thus influencing biochemical reactions inside the cells. The Protein Kinase C family (PKCs) is a group of serine/threonine kinases that are dependent on calcium (Ca(2+)), diacylglycerol, and phospholipids. Signaling pathways that induce variations on the levels of PKC activators have been implicated in the regulation of diverse cellular functions and, in turn, PKCs are key regulators of a plethora of cellular processes, including proliferation, differentiation, and tumorigenesis. Importantly, PKCs contain regions, both in the N-terminal regulatory domain and in the C-terminal catalytic domain, that are susceptible to redox modifications. In several pathophysiological conditions when the balance between oxidants, antioxidants, and alkylants is compromised, cells undergo redox stress. PKCs are cell-signaling proteins that are particularly sensitive to redox stress because modification of their redox-sensitive regions interferes with their activity and, thus, with their biological effects. In this review, we summarize the involvement of PKCs in health and disease and the importance of redox signaling in the regulation of this family of kinases.

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.

New protein kinase C activator regulates amyloid precursor protein processing in vitro by increasing alpha-secretase activity

The beta amyloid (Abeta) cascade has been at the forefront of the hypothesis used to describe the pathogenesis of Alzheimer's disease (AD). It is generally accepted that drugs that can regulate the processing of the amyloid precursor protein (APP) toward the non-amyloidogenic pathway may have a therapeutic potential. Previous studies have shown that protein kinase C (PKC) hypofunction has an important role in AD pathophysiology. Therefore, the effects of a new PKC activator, alpha-APP modulator [(2S,5S)-(E,E)-8-(5-(4-(trifluoromethyl)phenyl)-2,4-pentadienoylamino)benzolactam (TPPB)], on APP processing were investigated. Using PC12 cells and SH-SY5Y(APP695) cells, it was found that TPPB promoted the secretion of sAPPalpha without affecting full-length expression of APP. The increase in sAPPalpha by TPPB was blocked by inhibitors of PKC, extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK) and tyrosine kinase, suggesting the involvement of these signal transduction pathways. TPPB increased alpha-secretase activity [a disintegrin and metalloproteinase (ADAM)10 and 17], as shown by direct fluorescence activity detection and Western blot analysis. TPPB-induced sAPPalpha release was blocked by the metalloproteinase inhibitor TAPI-2, furin inhibitor CMK and by the protein-trafficking inhibitor brefeldin. The results also showed that TPPB decreased beta-secretase activity, Abeta40 release and beta site APP-cleaving enzyme 1 (BACE1) expression, but did not significantly affect neprilysin (NEP) and insulin-degrading enzyme (IDE) expression. Our data indicate that TPPB could direct APP processing towards the non-amyloidogenic pathway by increasing alpha-secretase activity, and suggest its therapeutic potential in AD.

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.

Mitogen activated protein kinase and protein kinase C activation mediate promotion of sAPPα secretion by deprenyl

The beta amyloid cascade plays a crucial role in the pathogenesis of Alzheimer's disease (AD). Therefore, drugs that regulate amyloid precursor protein (APP) processing toward the nonamyloidgenic pathway may have therapeutic potential. Many anti-dementia drugs can regulate APP processing in addition to their pharmacological properties. Deprenyl is a neuroprotective agent used to treat some neurodegenerative diseases, including AD. In the present study, the effects of deprenyl on APP processing were investigated. Using SK-N-SH and PC12 cells, it was demonstrated that deprenyl stimulated the release of the nonamyloidogenic alpha-secretase form of soluble APP (sAPPalpha) in a dose-dependent manner without affecting cellular APP expression. The increase of sAPPalpha secretion by deprenyl was blocked by the mitogen activated protein (MAP) kinase inhibitor U0126 and PD98059, and by the protein kinase C (PKC) inhibitor GF109203X and staurosporine, suggesting the involvement of these signal transduction pathways. Deprenyl induced phosphorylation of p42/44 MAP kinase, which was abolished by specific inhibitors of MAP kinase and PKC. Deprenyl also phosphorylated PKC and its major substrate, and myristoylated alanine-rich C kinase (MARCKS) at specific amino acid residues. The data also indicated that 10microM deprenyl successfully induced two PKC isoforms involved in the pathogenesis of AD, PKCalpha and PKCepsilon, to translocate from the cytosolic to the membrane fraction. This phenomenon was substantiated by immunocytochemistry staining. These data suggest a novel pharmacological mechanism in which deprenyl regulates the processing of APP via activation of the MAP kinase and PKC pathways, and that this mechanism may underlie the clinical efficacy of the drug in some AD patients.

Short-term interleukin-1β increases the release of secreted APPα via MEK1/2-dependent and JNK-dependent α-secretase cleavage in neuroglioma U251 cells

Several lines of neuroimmunological evidence correlate the development of the inflammatory responses of the brain with the formation of amyloid plaques associated with the pathogenesis of neurodegenerative disorders such as Alzheimer's disease. Within this context, we tested the ability of interleukin-1beta (IL-1beta) to regulate the processing of beta-amyloid precursor protein (beta-APP) in neuroglioma U251 cells. Our findings have shown that short-term treatment with IL-1beta (2 hr) resulted in a concentration-dependent decrease in the amount of the cell-associated form of beta-APP in U251 cells as compared to untreated cells, whereas a 2-hr treatment with IL-1beta led to increased release of secreted APP(alpha) fragment (sAPP(alpha)) into the conditioned media of the cells. The fact that sAPP(alpha) is an alpha-secretase cleavage metabolite of the cell-associated form of beta-APP, and the observation that IL-1beta-induced sAPP(alpha) release could be blocked by tissue inhibitors of metalloproteinases-1 (alpha-secretase inhibitors), suggested that alpha-secretase might be involved in IL-1beta-induced-sAPP(alpha) release. Moreover, to determine whether an intracellular signaling pathway mediates the IL-1beta-induced increase in sAPP(alpha) secretion, we used various specific signaling inhibitors and found that sAPP(alpha) release is significantly blocked by the mitogen-activated protein kinase (MEK1/2) inhibitor PD98059 and the c-Jun N-terminal kinase inhibitor SP600125. These findings suggested that the mechanism of IL-1beta-induced-sAPP(alpha) release is dependent on MEK1/2- and JNK-activated alpha-secretase cleavage in neuroglioma U251 cells.

Amyloid Pathology and Protein Kinase C (PKC): Possible Therapeutics Effects of PKC Activators

Amyloid beta-protein (Abeta) is one of the most studied peptides in human neurodegenerative disorders. Although much has been learned about the biochemistry of this peptide, fundamental questions such as when and how the Abeta becomes pathologic remain unanswered. In this article we review the recent findings on the biology and pathology of Abeta and the role protein kinase C (PKC) plays in these processes. The potential neuroprotective role of PKC and the possible therapeutic effects of PKC activators in Alzheimer's disease (AD) will be discussed. Briefly, comments will be also addressed on the role of PKC in cell death and neurogenesis in AD.

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.

Soluble oligomeric Aβ disrupts the protein kinase C signaling pathway

Alzheimer's disease (AD) is characterized by selective neurodegeneration of neurons involved in cognitive function. Current hypothesis for AD etiology needs to be reconsidered because fibrillar Abeta cannot explain selective neurodegeneration. Recent evidence suggests oligomeric Abeta may be more relevant to AD etiology. Here we show signaling disruption induced by oligomeric Abeta. Using the MTT assay, NT2 showed greatest susceptibility to soluble oligomeric Abeta. In the kinase assay, treatment with either monomeric Abeta or fibrillar Abeta evoked no response in PKA, PKC and TK. Oligomeric Abeta treatment, however, inactivated membranous PKC but activated cytosolic PKC in NT2 within 24 h. Our data suggest that oligomeric Abeta may cause selective neurodegeneration through a PKC signaling, distinctive from fibrillar Abeta.

Pro-apoptotic signaling in neuronal cells following iron and amyloid beta peptide neurotoxicity

In a previous report, we characterized several oxidative stress parameters during the course of amyloid beta (Abeta) peptide/Fe2+-induced apoptotic death in neuronal cells. In extending these findings, we now report a marked decrease in protein kinase C (PKC) isoforms, reduced Akt serine/threonine kinase activity, Bcl 2-associated death promoter (BAD) phosphorylation and enhanced p38 mitogen-activated protein kinase (MAPK) and caspase-9 and -3 activation, 12 h after addition of both 5 micro m Abeta and 5 micro m Fe2+. These activities reminiscent for a pro-apoptotic cellular course were blocked in the presence of the iron chelator deferroxamine. Abeta alone, increased PKC isoform levels between three- and four-fold after 12 h, enhanced Akt activity approximately eight-fold and Ser136 BAD phosphorylation two-fold, suggesting that by itself is not toxic. Fe2+ alone transiently enhanced p38 MAPK and caspase-9 and -3 enzymes indicative for cell damage, but was not sufficient to cause cell death as previously indicated. GF, a PKC inhibitor or wortmannin, a blocker of the Akt pathway enhanced Abeta/Fe2+-induced toxicity, while SB, a p38 MAPK inhibitor, prevented cell damage and apoptosis. These findings further support the hypothesis that metal ion chelation and inhibitors of pro-apoptotic kinase cascades may be beneficial for Alzheimer's disease therapy.

Guinea pigs as a nontransgenic model for APP processing in vitro and in vivo

Alzheimer's disease (AD) is characterized, amongst others, by the appearance of vascular and parenchymal beta-amyloid deposits in brain. Such aggregates are mainly composed of beta-amyloid peptides, which are derived by proteolytic processing of a larger amyloid precursor protein (APP). APP is highly conserved among mammalian species, but experimental studies in rodents are often hampered by the humble APP-processing in the amyloidogenic pathway and by the inability of rodent beta-amyloid peptides to form higher molecular aggregates such as soluble oligomers and insoluble beta-amyloid plaques. Thus, there is need for in vitro and in vivo model systems that allow identification of factors that increase amyloidogenic APP processing and accelerate beta-amyloid plaque formation and testing the potency of pharmacological manipulations to ameliorate beta-amyloid load in brain. Transgenic mice that overexpress human APP containing AD-associated mutations that favor the amyloidogenic pathway of APP processing represent such a model. However, mutations of the APP gene are not frequent in AD and, therefore, the mechanisms of beta-amyloid plaque formation, the composition of beta-amyloid plaques, and the accompanying tissue response in brain of these animals may be different from that in AD. In contrast, guinea pigs express beta-amyloid peptides of the human sequence and appear to represent a more physiological model to examine the long-term effects of experimental manipulations on APP processing and beta-amyloid plaque formation in vivo. Additionally, APP processing in guinea pig primary neuronal cultures has been shown to be similar to cultures of human origin. In this article we highlight the advantages and limitations of using guinea pigs as experimental models to study APP processing.

Human amyloid-beta causes changes in the levels of endothelial protein kinase C and its alpha isoform in vitro

Amyloid-beta (A(beta)) deposits and neurofibrillary pathology are characteristic features of Alzheimer's disease (AD). The association of A(beta) with cerebral vessels is an intriguing feature of AD. While there is considerable evidence of altered activities of the major isoforms of protein kinase C (PKC) in the vasculature and neurons of AD brains, little is known about the relationship between the Abeta toxicity and the altered PKC levels in cerebral endothelial cells. In this study, cultured brain endothelial cells exposed to A(beta)1-40 revealed a translocation of PKC from the membrane fraction to the cytosol. The content of the isoform PKC(alpha), involved in the regulation of amyloid precursor protein (APP) secretion, was decreased in the membrane-bound fraction of rat endothelial cells and increased in the cytosol after A(beta)1-40 treatment. These data suggest that the accumulation of A(beta) peptide in the cerebral vasculature may play a significant role in the down-regulation of PKC seen in the AD cerebral vasculature.