p75 and TrkA receptor signaling independently regulate amyloid precursor protein mRNA expression, isoform composition, and protein secretion in PC12 cells

New perspectives on the basal forebrain cholinergic system in Alzheimer's disease

The basal forebrain cholinergic system (BFCS) has long been implicated in age-related cognitive changes and the pathophysiology of Alzheimer's disease (AD). Limitations of cholinergic interventions helped to inspire a shift away from BFCS in AD research. A resurgence in interest in the BFCS following methodological and analytical advances has resulted in a call for the BFCS to be examined in novel frameworks. We outline the basic structure and function of the BFCS, its role in supporting cognitive and affective function, and its vulnerability to aging and AD. We consider the BFCS in the context of the amyloid hypothesis and evolving concepts in AD research: resilience and resistance to pathology, selective neuronal vulnerability, trans-synaptic pathology spread and sleep health. We highlight 1) the potential role of the BFCS in cognitive resilience, 2) recent work refining understanding about the selective vulnerability of BFCS to AD, 3) BFCS connectivity that suggests it is related to tau spreading and neurodegeneration and 4) the gap between BFCS involvement in AD and sleep-wake cycles.

A rare loss-of-function variant of ADAM17 is associated with late-onset familial Alzheimer disease

Common variants of about 20 genes contributing to AD risk have so far been identified through genome-wide association studies (GWAS). However, there is still a large proportion of heritability that might be explained by rare but functionally important variants. One of the so far identified genes with rare AD causing variants is ADAM10. Using whole-genome sequencing we now identified a single rare nonsynonymous variant (SNV) rs142946965 [p.R215I] in ADAM17 co-segregating with an autosomal-dominant pattern of late-onset AD in one family. Subsequent genotyping and analysis of available whole-exome sequencing data of additional case/control samples from Germany, UK, and USA identified five variant carriers among AD patients only. The mutation inhibits pro-protein cleavage and the formation of the active enzyme, thus leading to loss-of-function of ADAM17 alpha-secretase. Further, we identified a strong negative correlation between ADAM17 and APP gene expression in human brain and present in vitro evidence that ADAM17 negatively controls the expression of APP. As a consequence, p.R215I mutation of ADAM17 leads to elevated Aß formation in vitro. Together our data supports a causative association of the identified ADAM17 variant in the pathogenesis of AD.

Sphingolipids in neurodegeneration (with focus on ceramide and S1P)

For many decades, research on sphingolipids associated with neurodegenerative disease focused on alterations in glycosphingolipids, particularly glycosylceramides (cerebrosides), sulfatides, and gangliosides. This seemed quite natural since many of these glycolipids are constituents of myelin and accumulated in lipid storage diseases (sphingolipidoses) resulting from enzyme deficiencies in glycolipid metabolism. With the advent of recognizing ceramide and its derivative, sphingosine-1-phosphate (S1P), as key players in lipid cell signaling and regulation of cell death and survival, research focus shifted toward these two sphingolipids. Ceramide and S1P are invoked in a plethora of cell biological processes participating in neurodegeneration such as ER stress, autophagy, dysregulation of protein and lipid transport, exosome secretion and neurotoxic protein spreading, neuroinflammation, and mitochondrial dysfunction. Hence, it is timely to discuss various functions of ceramide and S1P in neurodegenerative disease and to define sphingolipid metabolism and cell signaling pathways as potential targets for therapy.

Exercise-induced brain-derived neurotrophic factor expression: Therapeutic implications for Alzheimer's dementia

Emerging evidence indicates that moderate intensity aerobic exercise is positively correlated with cognitive function and memory. However, the exact mechanisms underlying such improvements remain unclear. Recent research in animal models allows proposition of a pathway in which brain-derived neurotrophic factor (BDNF) is a key mediator. This perspective draws upon evidence from animal and human studies to highlight such a mechanism whereby exercise drives synthesis and accumulation of neuroactive metabolites such as myokines and ketone bodies in the periphery and in the hippocampus to enhance BDNF expression. BDNF is a neurotrophin with well-established properties of promoting neuronal survival and synaptic integrity, while its influence on energy transduction may provide the crucial link between inherent vascular and metabolic benefits of exercise with enhanced brain function. Indeed, BDNF mRNA and protein is robustly elevated in rats following periods of voluntary exercise. This was also correlated with improved spatial memory, while such benefits were abolished upon inhibition of BDNF signaling. Similarly, both BDNF and cardiovascular fitness arising from aerobic exercise have been positively associated with hippocampal volume and function in humans. We postulate that exercise will attenuate cortical atrophy and synaptic loss inherent to neurodegenerative disorders - many of which also exhibit aberrant down-regulation of BDNF. Thus, the proposed link between BDNF, exercise and cognition may have critical therapeutic implications for the prevention and amelioration of memory loss and cognitive impairment in Alzheimer's disease and associated dementias.

Association of TrkA and APP Is Promoted by NGF and Reduced by Cell Death-Promoting Agents

The amyloid precursor protein (APP) interacts with the tropomyosin receptor kinase A (TrkA) in normal rat, mouse, and human brain tissue but not in Alzheimer's disease (AD) brain tissue. However, it has not been reported whether the two proteins interact directly, and if so, which domains are involved. Clarifying these points will increase our understanding of the role and regulation of the TrkA/APP interaction in normal brain functioning as well as in AD. Here we addressed these questions using bimolecular fluorescence complementation (BiFC) and the proximity ligation assay (PLA). We demonstrated that exogenously expressed APP and TrkA associate through their juxtamembrane/transmembrane domains, to form a complex that localizes mainly to the plasma membrane, endoplasmic reticulum (ER) and Golgi. Formation of the complex was inhibited by p75NTR, ShcC and Mint-2. Importantly, we demonstrated that the association between endogenous APP and TrkA in primary septal neurons were modified by NGF, or by drugs that either inhibit ER-to-Golgi transport or perturb microtubules and microfilaments. Interestingly, several agents that induce cell death [amyloid β (Aβ)-peptide, staurosporine and rapamycin], albeit via different mechanisms, all caused dissociation of APP/TrkA complexes and increased production of C-terminal fragment (β-CTF) APP fragment. These findings open new perspectives for investigating the interplay between these proteins during neurodegeneration and AD.

Inflammation Combined with Ischemia Produces Myelin Injury and Plaque-Like Aggregates of Myelin, Amyloid-β and AβPP in Adult Rat Brain

Background: Ischemia, white matter injury, and Alzheimer’s disease (AD) pathologies often co-exist in aging brain. How one condition predisposes to, interacts with, or perhaps causes the others remains unclear.

Objectives: To better understand the link between ischemia, white matter injury, and AD, adult rats were administered lipopolysaccharide (LPS) to serve as an inflammatory stimulus, and 24 h later subjected to 20-min focal cerebral ischemia (IS) followed by 30-min hypoxia (H).

Methods: Myelin and axonal damage, as well as amyloid-β (Aβ) and amyloid-β protein precursor (AβPP) deposition were examined by Western blot and immunocytochemistry following LPS/IS/H. Findings were compared to the 5XFAD mouse AD brain.

Results: Myelin/axonal injury was observed bilaterally in cortex following LPS/IS/H, along with an increase in IL-1, granzyme B, and LPS. AβPP deposition was present in ischemic striatum in regions of myelin loss. Aβ_1-42 and AβPP were deposited in small foci in ischemic cortex that co-localized with myelin aggregates. In the 5XFAD mouse AD model, cortical amyloid plaques also co-localized with myelin aggregates.

Conclusions: LPS/IS/H produce myelin injury and plaque-like aggregates of myelin. AβPP and Aβ co-localize with these myelin aggregates.

Enhancement of the nonamyloidogenic pathway by exogenous NGF in an Alzheimer transgenic mouse model

Nerve growth factor (NGF) is an important nerve cell growth regulatory factor and has an indispensable role in the development, survival and regeneration of the cholinergic basal forebrain (CBF) neurons, and it has multiple targets when used for Alzheimer's Disease (AD) therapy. In this study, we observed whether NGF can affect cholinergic neurons to change amyloid-β precursor protein (APP) metabolism process and reduce amyloidosis in AD brains. NGF was administered intranasally to APP/PS1 double-transgenic mice for 14weeks. We observed an increase in APP695 and ADAM10 and a decrease in BACE1 and PS1 protein levels and, subsequently, a reduction in Aβ1-40 and Aβ1-42 levels and Aβ burden were present in NGF-treated mice brains, suggesting that NGF enhanced the APP nonamyloidogenic cleavage pathway and reduced the Aβ generation in the APP/PS1 transgenic mice brains.

Inhibition of nerve growth factor-induced neurite outgrowth from PC12 cells by dexamethasone: signaling pathways through the glucocorticoid receptor and phosphorylated Akt and ERK1/2

Glucocorticoids are important mediators of the stress response and are commonly employed as drugs for the suppression of immune rejection after organ transplantation. Previous investigations uncovered the possibility of mood depression in patients undergoing long-term treatment with synthetic glucocorticoids, including dexamethasone (DEX). Exogenous glucocorticoids and their synthetic derivatives can also adversely affect the development of the central nervous system. Although neurite extension from rat pheochromocytoma-derived PC12 cells and a variety of primary neurons is stimulated by nerve growth factor (NGF), and signaling pathways triggered by the binding of NGF to tyrosine kinase receptor type 1 (TrkA) function in both neurite outgrowth and neuronal survival, the effect of DEX on the activation of regulatory proteins and pathways downstream of TrkA has not been well characterized. To analyze the influence of DEX on NGF-induced neurite outgrowth and signaling, PC12 cells, a widely utilized model of neuronal differentiation, were pretreated with the glucocorticoid prior to NGF induction. NGF-induced neurite outgrowth was attenuated by pretreatment with DEX, even in the absence of DEX after the addition of NGF. Moreover, DEX suppressed the phosphorylation of Akt and extracellular-regulated kinase 1/2 (ERK1/2) in the neurite outgrowth signaling cascade initiated by NGF. Finally, the glucocorticoid receptor (GR) antagonist, RU38486, counteracted the inhibitory effect of DEX pretreatment, not only on the phosphorylation of Akt and ERK1/2, but also on neurite extension from PC12 cells. These results suggest that DEX binding to the GR impairs NGF-promoted neurite outgrowth by interfering with the activation/phosphorylation of Akt and ERK1/2. These novel findings are likely to be useful for elucidating the central nervous system depressive mechanism(s) of action of DEX and other glucocorticoids.

Soluble β-amyloid Precursor Protein Alpha binds to p75 neurotrophin receptor to promote neurite outgrowth

Background

The cleavage of β-amyloid precursor protein (APP) generates multiple proteins: Soluble β-amyloid Precursor Protein Alpha (sAPPα), sAPPβ, and amyloid β (Aβ). Previous studies have shown that sAPPα and sAPPβ possess neurotrophic properties, whereas Aβ is neurotoxic. However, the underlying mechanism of the opposing effects of APP fragments remains poorly understood. In this study, we have investigated the mechanism of sAPPα-mediated neurotrophic effects. sAPPα and sAPPβ interact with p75 neurotrophin receptor (p75^NTR), and sAPPα promotes neurite outgrowth.

Methods and Findings

First, we investigated whether APP fragments interact with p75^NTR, because full-length APP and Aβ have been shown to interact with p75^NTR in vitro. Both sAPPα and sAPPβ were co-immunoprecipitated with p75^NTR and co-localized with p75^NTR on COS-7 cells. The binding affinity of sAPPα and sAPPβ for p75^NTR was confirmed by enzyme-linked immunosorbent assay (ELISA). Next, we investigated the effect of sAPPα on neurite outgrowth in mouse cortical neurons. Neurite outgrowth was promoted by sAPPα, but sAPPα was uneffective in a knockdown of p75^NTR.

Conclusion

We conclude that p75^NTR is the receptor for sAPPα to mediate neurotrophic effects.

Nerve growth factor: from the early discoveries to the potential clinical use

The physiological role of the neurotrophin nerve growth factor (NGF) has been characterized, since its discovery in the 1950s, first in the sensory and autonomic nervous system, then in central nervous, endocrine and immune systems. NGF plays its trophic role both during development and in adulthood, ensuring the maintenance of phenotypic and functional characteristic of several populations of neurons as well as immune cells. From a translational standpoint, the action of NGF on cholinergic neurons of the basal forebrain and on sensory neurons in dorsal root ganglia first gained researcher's attention, in view of possible clinical use in Alzheimer's disease patients and in peripheral neuropathies respectively. The translational and clinical research on NGF have, since then, enlarged the spectrum of diseases that could benefit from NGF treatment, at the same time highlighting possible limitations in the use of the neurotrophin as a drug. In this review we give a comprehensive account for almost all of the clinical trials attempted until now by using NGF. A perspective on future development for translational research on NGF is also discussed, in view of recent proposals for innovative delivery strategies and/or for additional pathologies to be treated, such as ocular and skin diseases, gliomas, traumatic brain injuries, vascular and immune diseases.

Effects of nerve growth factor and nitric oxide synthase inhibitors on amyloid precursor protein mRNA levels and protein stability

We determined previously that nitric oxide (NO) modulates the nerve growth factor (NGF)-mediated increases in amyloid precursor protein (APP) levels in PC12 cells. To elucidate potential mechanisms, the effects of NGF and NO synthase (NOS) inhibitors on APP mRNA levels and protein stability were evaluated. Surprisingly, treatment of PC12 cells with NGF resulted in decreased levels of APP695 and APP751/770 mRNA. Therefore, the effect of NGF on APP protein stability was examined using the translation inhibitor, cycloheximide. Under these conditions, NGF did not alter the rate of APP degradation, suggesting that NGF may be enhancing the translation rate of APP. Since NOS inhibitors attenuate the NGF-mediated increase in APP levels, their effect on APP mRNA levels and protein stability was also assessed. S-methylisothiourea (S-MIU), selective for inducible NOS, decreased both APP695 and APP751/770 mRNA levels while the non-selective NOS inhibitor, N(ω)-nitro-L-arginine methylester (L-NAME) had no effect. In both control and NGF-treated PC12 cells, S-MIU increased the half-life of APP, with the greatest effect observed with the APP695 isoform. Based on these data we propose that in PC12 cells, NGF increases APP levels through enhanced translation rate and that NO, which modulates the NGF-induced increase in APP protein, also regulates APP mRNA levels and could play a role in APP processing.

Apoptosis and in vitro Alzheimer disease neuronal models

Alzheimer disease (AD) is a human neurodegenerative disease characterized by co-existence of extracellular senile plaques (SP) and neurofibrillary tangles (NFT) associated with an extensive neuronal loss, primarily in the cerebral cortex and hippocampus. Several studies suggest that caspase(s)-mediated neuronal death occurs in cellular and animal AD models as well as in human brains of affected patients, although an etiologic role of apoptosis in such neurodegenerative disorder is still debated. This review summarizes the experimental evidences corroborating the possible involvement of apoptosis in AD pathogenesis and discusses the usefulness of ad hoc devised in vitro approaches to study how caspase(s), amyloidogenic processing and tau metabolism might reciprocally interact leading to neuronal death.

Activation of amyloid precursor protein processing by growth factors is dependent on Ras GTPase activity

The β-amyloid peptide is generated by the proteolysis of the amyloid precursor protein (APP) by the action of β- and γ-secretase. The mechanisms underlying this process are poorly understood. Using a cell-based reporter gene assay we analysed the possible signals and pathways that could be involved in APP cleavage. We used the stable cell line HeLa AG that expresses the human APP(695) fused with the yeast transcription factor Gal4. This fusion protein is normally translocated into the plasma membrane and after APP-Gal4 cleavage, the AICD-Gal4 fragment released can activate the transcription of a luciferase reporter gene. Through this reporter system, we demonstrated that Ras GTPase, but not Ral and Rap, could promote APP-Gal4 cleavage. In addition HeLa AG cells stimulated with EGF or PDGF or overexpressing EGFR exhibit increased APP proteolysis in a Ras-dependent way. This process is also dependent on γ-secretase activity, being abolished by the γ-secretase inhibitor DAPT.

Nerve growth factor attenuates oxidant-induced β-amyloid neurotoxicity in sporadic Alzheimer's disease cybrids

Although mitochondrial dysfunction has been linked to Alzheimer's disease (AD), it is not fully understood how this dysfunction may induce neuronal death. In this study, we show that transmitochondrial hybrid cells (cybrids) expressing mitochondrial genes from patients with sporadic AD (SAD) have substantial alterations in basal upstream tyrosine kinase signaling and downstream serine-threonine kinase signaling that are mediated by intracellular free radicals. This is associated with reduced tropomyocin receptor kinase (TrkA) and p75 neurotrophin receptor receptor expression that profoundly alters nerve growth factor signaling, increases generation of Aβ and decreases viability. Many of these observed effects in SAD cybrids would be predicted to increase risk of premature neuronal death and reduce resistance to stressors and add further support for the pathogenic role of mtDNA expression in the pathogenesis of SAD.

Nerve growth factor as a paradigm of neurotrophins related to Alzheimer's disease

Converging lines of evidence on the possible connection between NGF signaling and Alzheimer's diseases (AD) are unraveling new facets which could depict this neurotrophin (NTF) in a more central role. AD animal models have provided evidence that a shortage of NGF supply may induce an AD-like syndrome. In vitro experiments, moreover, are delineating a possible temporal and causal link between APP amiloydogenic processing and altered post-translational tau modifications. After NGF signaling interruption, the pivotal upstream players of the amyloid cascade (APP, beta-secretase, and active form of gamma-secretase) are up-regulated, leading to an increased production of amyloid beta peptide (Abeta) and to its intracellular aggregation in molecular species of different sizes. Contextually, the Abeta released pool generates an autocrine toxic loop in the same healthy neurons. At the same time tau protein undergoes anomalous, GSKbeta-mediated, phosphorylation at specific pathogenetic sites (Ser262 and Thr 231), caspase(s) and calpain- I- mediated truncation, detachment from microtubules with consequent cytoskeleton collapse and axonal transport impairment. All these events are inhibited when the amyloidogenic processing is reduced by beta and gamma secretase inhibitors or anti-Abeta antibodies and appear to be causally correlated to TrkA, p75CTF, Abeta, and PS1 molecular association in an Abeta-mediated fashion. In this scenario, the so-called trophic action exerted by NGF (and possibly also by other neurotrophins) in these targets neurons is actually the result of an anti-amyloidogenic activity.

Reduction of beta-amyloid levels by novel protein kinase C(epsilon) activators

Isoform-specific protein kinase C (PKC) activators may be useful as therapeutic agents for the treatment of Alzheimer disease. Three new epsilon-specific PKC activators, made by cyclopropanation of polyunsaturated fatty acids, have been developed. These activators, AA-CP4, EPA-CP5, and DHA-CP6, activate PKCepsilon in a dose-dependent manner. Unlike PKC activators that bind to the 1,2-diacylglycerol-binding site, such as bryostatin and phorbol esters, which produce prolonged down-regulation, the new activators produced sustained activation of PKC. When applied to cells expressing human APPSwe/PS1delta, which produce large quantities of beta-amyloid peptide (Abeta), DCP-LA and DHA-CP6 reduced the intracellular and secreted levels of Abeta by 60-70%. In contrast to the marked activation of alpha-secretase produced by PKC activators in fibroblasts, the PKC activators produced only a moderate and transient activation of alpha-secretase in neuronal cells. However, they activated endothelin-converting enzyme to 180% of control levels, suggesting that the Abeta-lowering ability of these PKCepsilon activators is caused by increasing the rate of Abeta degradation by endothelin-converting enzyme and not by activating nonamyloidogenic amyloid precursor protein metabolism.

Synaptic degeneration in Alzheimer's disease

Synaptic loss is the major neurobiological substrate of cognitive dysfunction in Alzheimer's disease (AD). Synaptic failure is an early event in the pathogenesis that is clearly detectable already in patients with mild cognitive impairment (MCI), a prodromal state of AD. It progresses during the course of AD and in most early stages involves mechanisms of compensation before reaching a stage of decompensated function. This dynamic process from an initially reversible functionally responsive stage of down-regulation of synaptic function to stages irreversibly associated with degeneration might be related to a disturbance of structural brain self-organization and involves morpho-regulatory molecules such as the amyloid precursor protein. Further, recent evidence suggests a role for diffusible oligomers of amyloid beta in synaptic dysfunction. To form synaptic connections and to continuously re-shape them in a process of ongoing structural adaptation, neurons must permanently withdraw from the cell cycle. Previously, we formulated the hypothesis that differentiated neurons after having withdrawn from the cell cycle are able to use molecular mechanisms primarily developed to control proliferation alternatively to control synaptic plasticity. The existence of these alternative effector pathways within neurons might put them at risk of erroneously converting signals derived from plastic synaptic changes into the program of cell cycle activation, which subsequently leads to cell death. The molecular mechanisms involved in cell cycle activation might, thus, link aberrant synaptic changes to cell death.

Tyrosine kinase nerve growth factor receptor switches from prosurvival to proapoptotic activity via Abeta-mediated phosphorylation

The present study shows that increased Abeta production in hippocampal neurons, due to a failure of NGF signal, induces an unexpected phosphorylation of tyrosine kinase receptor A (TrkA), followed by activation of the phospholipase C gamma (PLCgamma) pathway and neuronal death. Such phosphorylation seems causally connected with 2 kinases known be involved in amyloidogenesis, Src and CDK5, and associated with alpha and gamma secretase-mediated p75 processing. Pharmacologic inhibition of TrkA phosphorylation and partial silencing of TrkA and/or p75 receptors prevent PLCgamma activation and protect neurons from death. Concomitantly with these events, TrkA, p75, Abeta peptides, and PS1 protein coimmunoprecipitate, suggesting their direct interplay in the subsequent onset of apoptotic death. Together, these findings depict a cellular mechanism whereby the same cellular transducing system may invert its intracellular message from trophic and antiapoptotic to a death signaling, which could also have relevance in the onset of Alzheimer's disease.

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

RATIONALE

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSION

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

Neurotrophic factors in Alzheimer's disease: role of axonal transport

Neurotrophic factors (NTF) are small, versatile proteins that maintain survival and function to specific neuronal populations. In general, the axonal transport of NTF is important as not all of them are synthesized at the site of its action. Nerve growth factor (NGF), for instance, is produced in the neocortex and the hippocampus and then retrogradely transported to the cholinergic neurons of the basal forebrain. Neurodegenerative dementias like Alzheimer’s disease (AD) are linked to deficits in axonal transport. Furthermore, they are also associated with imbalanced distribution and dysregulation of NTF. In particular, brain-derived neurotrophic factor (BDNF) plays a crucial role in cognition, learning and memory formation by modulating synaptic plasticity and is, therefore, a critical molecule in dementia and neurodegenerative diseases. Here, we review the changes of NTF expression and distribution (NGF, BDNF, neurotrophin-3, neurotrophin-4/5 and fibroblast growth factor-2) and their receptors [tropomyosin-related kinase (Trk)A, TrkB, TrkC and p75^NTR] in AD and AD models. In addition, we focus on the interaction with neuropathological hallmarks Tau/neurofibrillary tangle and amyloid-β (Abeta)/amyloid plaque pathology and their influence on axonal transport processes in order to unify AD-specific cholinergic degeneration and Tau and Abeta misfolding through NTF pathophysiology.

Beta/A4-Amyloid increases nerve growth factor production in rat primary hippocampal astrocyte cultures

Nerve growth factor (NGF), a member of the neurotrophin family, is an essential mediator of neuronal activity and synaptic plasticity of basal forebrain cholinergic neurons (BFCN). In processes of chronic degeneration of BFCN like in Alzheimer's disease (AD), characterized among others by amyloid containing plaques, NGF has been shown to improve cognitive decline and rescue BFCN but also to reduce survival of hippocampal neurons via p75 neurotrophin receptor (p75). Little is known about the mechanisms of NGF regulation in glial cells under pathological conditions in AD. This study investigates the influence of amyloid administration on the NGF protein secretion in rat primary hippocampal astrocytes. Astrocytes were stimulated with "aged" beta/A4-Amyloid (1-40), and NGF was measured in different fractions, such as supernatant, vesicles, and cytosol fraction. Treatment with amyloid at a final concentration of 10 microM for 72 h led to increased NGF protein levels up to 30-fold increase compared to unstimulated controls. This observation may be an endogenous neuroprotective mechanism possibly contributing to a delay of amyloid-dependent loss of cholinergic neurons or contribute to accelerated neuronal death by activation of p75 within Alzheimer pathology.

Nitric oxide synthase inhibitors modulate nerve growth factor-mediated regulation of amyloid precursor protein expression in PC12 cells

Nerve growth factor (NGF) can regulate nitric oxide synthase (NOS) expression and nitric oxide (NO) can modulate NGF-mediated neurotrophic responses. In this study, the role of NO in NGF-stimulated amyloid precursor protein (APP) levels was studied. PC12 cells were treated with either the non-selective NOS inhibitor N(omega)-nitro-L-arginine methylester (L-NAME) or the inducible NOS selective inhibitor s-methylisothiourea (S-MIU), and the effect on NGF-mediated increases in APP expression was determined. NGF significantly increased total APP protein levels following 96 h of treatment and this increase was prevented in cells pre-treated with S-MIU. Pre-treatment of cells with actinomycin D also blocked this NGF-mediated induction of APP, indicating de novo protein synthesis is necessary. Treatment with NGF increased APP promoter activity; however, this increase was only partially inhibited by pre-treatment with S-MIU and was increased in the presence of L-NAME. This suggests that NO may be modulating other aspects of APP expression in addition to transcription. Inhibition of NGF signaling pathways was also investigated using inhibitors of mitogen-activated protein (MAP) kinase (U0126), Akt (LY294002) and protein kinase C (PKC; U73122 and bisindolylmaleimide 1 (BIS-1)) activation. Inhibition of each of these pathways prevented NGF-mediated increases in APP protein expression; however, only BIS-1 attenuated NGF-mediated increases in promoter activation. This study indicates that NO is involved in the NGF-mediated regulation of APP, in part at the level of APP transcription and could involve the modulation of NGF signal transduction pathways.

Receptor tyrosine kinases positively regulate BACE activity and Amyloid-β production through enhancing BACE internalization

Amyloid-beta (Abeta) peptide, the primary constituent of senile plaques in Alzheimer's disease (AD), is generated by beta-secretase- and gamma-secretase-mediated sequential proteolysis of the amyloid precursor protein (APP). The aspartic protease, beta -site APP cleavage enzyme (BACE), has been identified as the main beta-secretase in brain but the regulation of its activity is largely unclear. Here, we demonstrate that both BACE activity and subsequent Abeta production are enhanced after stimulation of receptor tyrosine kinases (RTKs), such as the receptors for epidermal growth factor (EGF) and nerve growth factor (NGF), in cultured cells as well as in mouse hippocampus. Furthermore, stimulation of RTKs also induces BACE internalization into endosomes and Golgi apparatus. This enhancement of BACE activity and Abeta production upon RTK activation could be specifically inhibited by Src family kinase inhibitors and by depletion of endogenous c-Src with RNAi, and could be mimicked by over-expressed c-Src. Moreover, blockage of BACE internalization by a dominant negative form of Rab5 also abolished the enhancement of BACE activity and Abeta production, indicating the requirement of BACE internalization for the enhanced activity. Taken together, our study presents evidence that BACE activity and Abeta production are under the regulation of RTKs and this is achieved via RTK-stimulated BACE internalization, and suggests that an aberration of such regulation might contribute to pathogenic Abeta production.

The modular systems biology approach to investigate the control of apoptosis in Alzheimer's disease neurodegeneration

Apoptosis is a programmed cell death that plays a critical role during the development of the nervous system and in many chronic neurodegenerative diseases, including Alzheimer's disease (AD). This pathology, characterized by a progressive degeneration of cholinergic function resulting in a remarkable cognitive decline, is the most common form of dementia with high social and economic impact. Current therapies of AD are only symptomatic, therefore the need to elucidate the mechanisms underlying the onset and progression of the disease is surely needed in order to develop effective pharmacological therapies. Because of its pivotal role in neuronal cell death, apoptosis has been considered one of the most appealing therapeutic targets, however, due to the complexity of the molecular mechanisms involving the various triggering events and the many signaling cascades leading to cell death, a comprehensive understanding of this process is still lacking.

Modular systems biology is a very effective strategy in organizing information about complex biological processes and deriving modular and mathematical models that greatly simplify the identification of key steps of a given process.

This review aims at describing the main steps underlying the strategy of modular systems biology and briefly summarizes how this approach has been successfully applied for cell cycle studies. Moreover, after giving an overview of the many molecular mechanisms underlying apoptosis in AD, we present both a modular and a molecular model of neuronal apoptosis that suggest new insights on neuroprotection for this disease.

Propentofylline attenuates tau hyperphosphorylation in Alzheimer's Swedish mutant model Tg2576

Key pathological hallmarks of Alzheimer's disease (AD) are the deposition of amyloid plaques containing Abeta-peptides and the formation of neurofibrillary tangles containing hyperphosphorylated tau. Propentofylline (PPF) is a synthetic xanthine derivative that inhibits phosphodiesterase and adenosine uptake. These effects of PPF influence many cellular functions including stimulating synthesis/release of nerve growth factor. We tested the effects of PPF on disease progression in transgenic mice overexpressing the Swedish mutant human APP (Tg2576). The untreated Tg mice show, together with increased amyloidogenesis, increased levels of tau hyperphosphorylation and increased ratios of the activated to inactivated GSK-3beta, one of the key kinases that can phosphorylate tau. One month of PPF feeding (40 mg/kg per day) reduced the burden of amyloid plaques and the levels of hyperphosphorylated tau and immunoreactive IL-1beta. In parallel with these changes, PPF reduced the activated form of GSK-3beta and increased the inactivated form of GSK-3beta, restoring their ratio almost to normal values. These results demonstrate that PPF can exert multiple protective effects on both amyloidogenesis and tau hyperphosphorylation in an animal model of AD. Our earlier report [Neurochem. Int. 43(3) (2003) 225] demonstrated that Tg2576 animals show decreased levels of mRNA for NGF with increased amyloid burden while feeding of PPF results in a major shift from beta-amyloidogenic to alpha-secretory processing of APP together with increased expression of NGF mRNA. The current new data enlarge our understanding of PPF effects in brain and of tau hyperphosphorylation in Tg animals and are consistent with the hypothesis that GSK-3beta is a nodal point linking amyloid and tau pathology. Therapeutic interventions directed toward multiple pathological processes may be more protective than treatments directed toward a single process. The new results reported here indicate that further testing of PPF as a potential therapy in AD is warranted.

Epithelial growth control by neurotrophins: leads and lessons from the hair follicle

Neurotrophins (NTs) exert many growth-regulatory functions beyond the nervous system. For example, murine hair follicles (HF) show developmentally and spatio-temporally stringently controlled expression of NTs, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and NT-4, and their cognate receptors, tyrosine kinase A-C (TrkA-C) and p75 neurotrophin receptor (p75NTR). Follicular NT and NT receptor expression exhibit significant, hair cycle-dependent fluctuations on the gene and protein level, which are mirrored by changes in nerve fiber density and neurotransmitter/neuropeptide content in the perifollicular neural networks. NT-3/TrkC and NGF/TrkA signaling stimulate HF development, while NT-3, NT-4 and BDNF inhibit the growth (anagen) of mature HF by the induction of apoptosis-driven HF regression (catagen). p75NTR stimulation inhibits HF development and stimulates catagen. Since the HF is thus both a prominent target and key peripheral source of NT, dissecting the role of NTs in the control of HF morphogenesis and cyclic remodeling provides a uniquely accessible, and easily manipulated, clinically relevant experimental model, which has many lessons to teach. Given that our most recent data also implicate NTs in human hair growth control, selective NT receptor agonists and antagonists may become innovative therapeutic tools for the management of hair growth disorders (alopecia, effluvium, hirsutism). Since, however, the same NT receptor agonists that inhibit hair growth (e.g., BDNF, NT-4) can actually stimulate epidermal keratinocyte proliferation, NT may exert differential effects on defined keratinocyte subpopulations. The studies reviewed here provide new clues to understanding the complex roles of NT in epithelial tissue biology and remodeling in vivo, and invite new applications for synthetic NT receptor ligands for the treatment of epithelial growth disorders, exploiting the HF as a lead model.

p75 and TrkA receptors are both required for uptake of NGF in adult sympathetic neurons: use of a novel fluorescent NGF conjugate

We have developed and tested the biological activity and specificity of a novel fluorescent dextran-Texas Red-nerve growth factor (DTR-NGF) conjugate. DTR-NGF was found to promote survival and neurite outgrowth in cultured dissociated sympathetic neurons similarly to native NGF. The conjugate was taken up and transported retrogradely by terminal sympathetic nerves innervating the iris to neurons in the ipsilateral superior cervical ganglion (SCG) of young adult rats. Uptake and transport was assessed by counting numbers of labelled neurons and by measuring intensity of neuronal labelling using confocal microscopy and image analysis. DTR-NGF labelling in SCG neurons was shown to be dose-dependent with an EC(50) of 75 ng. Similar concentrations of unconjugated DTR resulted in no neuronal labelling. DTR-NGF uptake was competed off using a 50-fold excess of native NGF, resulting in a 73% reduction in numbers of labelled neurons. Pretreatment of nerve terminals with function-blocking antibodies against the low (p75) and high (TrkA) affinity NGF receptors resulted in a large (85-93%) reduction in numbers of DTR-NGF labelled neurons. Anti-p75 and anti-TrkA antibodies had comparable effects which were concentration-dependent. These findings indicate that both receptors are required for uptake of NGF in adult rat sympathetic neurons. In particular, the results provide strong evidence that the p75 receptor plays a more active role in transducing the NGF signal than has been proposed.

Brain-derived neurotrophic factor stimulates beta-amyloid gene promoter activity by a Ras-dependent/AP-1-independent mechanism in SH-SY5Y neuroblastoma cells

The beta-amyloid peptide, the major component of Alzheimer-associated plaques, derives from a larger beta-amyloid precursor protein (APP), that is expressed in both neural and non-neural cells. Overexpression of APP actively contributes to the development of senile plaques and is considered a risk factor for the disease. APP expression is regulated by a variety of cellular mediators, among them ligands of tyrosine kinase receptors. In this study, we present evidence that brain-derived neurotrophic factor (BDNF) modulates, in a dose- and time-dependent fashion, APP promoter activity in SH-SY5Y neuroblastoma cells transiently expressing the receptor TrkB. The APP promoter contains two potential AP-1 sites, and we examined whether or not protein kinase C (PKC) and the AP-1 sites of the promoter mediate the BDNF-induced stimulation of APP. Stimulation of APP promoter activity by BDNF was not affected by the PKC inhibitor bisindolylmaleimide, or by dominant negative mutants of the AP-1 components Fos and Jun, which, however, blocked the response to phorbol esters. These results suggest that activation of the APP promoter by BDNF is largely independent of PKC and AP-1. In contrast, activated Ras increased APP promoter activity in SH-SY5Y cells, and a dominant negative mutant of Ras abolished BDNF-mediated promoter stimulation. Taken together, our results suggest a mechanism that involves activation of the Ras/MAP kinase signaling pathway, and phosphorylation of as yet unidentified effectors which in turn can activate response elements within the APP promoter.

Cyclooxygenase-2 stimulates production of amyloid beta-peptide in neuroblastoma x glioma hybrid NG108-15 cells

Cyclooxygenase (COX) synthesizes bioactive prostaglandins from arachidonic acid, and there are COX-1 and COX-2 isoforms with distinct pathophysiological functions. Recent studies demonstrated that COX-2 expression was up-regulated in the brain of patients with Alzheimer's disease. We established mouse neuroblastoma x rat glioma hybrid NG108-15 cells stably expressing human COX-2. The COX-2-expressing cells showed 3- to 4-fold increases in both COX activity and prostaglandin E(2) production. The mRNA level of amyloid precursor protein (APP) was elevated by approximately 2-fold in the COX-2-expressing cells compared with mock-transfected cells. Amyloid beta-peptide and a secreted form of APP, both derived from APP by proteolysis was also increased. Interestingly, neurite outgrowth was stimulated in the COX-2-expressing cells with concomitant reduction of the cell proliferation rate. A selective COX-2 inhibitor (JTE-522) and a nonselective COX inhibitor (indomethacin) suppressed production of amyloid beta-peptide and a secreted form of APP by inhibition of APP mRNA level, suggesting that COX-2 plays important roles in the neurodegenerative processes of Alzheimer's disease.

Nerve growth factor treatment in dementia

Millions of people are affected by Alzheimer disease. As longevity increases, so will the number of patients with dementia. This has led to an intense search for successful treatment strategies. One area of interest is neurotrophic factors. Brain development and neuronal maintenance, as well as protective efforts, are mediated by a large number of different neurotrophic factors acting on specific receptors. In neurodegenerative disorders, there may be a possibility of rescuing degenerating neurons and stimulating terminal outgrowth with use of neurotrophic factors. The first neurotrophic factor discovered was nerve growth factor (NGF). A wealth of animal studies have shown that cholinergic neurons are NGF sensitive and NGF dependent, which is especially interesting in cognitive disorders, in which central cholinergic projections are important for cognitive function. In Alzheimer disease, cholinergic neurons have been shown to degenerate. This suggests that NGF may be used to pharmacologically counteract cholinergic degeneration and/or induce terminal sprouting in Alzheimer disease. Data from animal studies, as well as from the author's recent clinical trial, in which NGF was infused to the lateral ventricle in patients with Alzheimer disease, will be presented. Effects of NGF on cognition, as well as issues regarding dosage, side effects, and alternative ways of administering NGF, will be discussed.

Regulation of Alzheimer beta-amyloid precursor trafficking and metabolism

Alzheimer's disease (AD) is characterized by the intracranial accumulation of the 4 kDa amyloid-beta peptide (Abeta), following proteolysis of a approximately 700-amino acid, integral membrane precursor, the Alzheimer amyloid precursor protein (APP). The best evidence causally linking APP to AD has been provided by the discovery of mutations within the APP coding sequence that segregate with disease phenotypes in autosomal dominant forms of familial AD (FAD). Though FAD is rare ( < 10% of all AD), the hallmark features (amyloid plaques, neurofibrillary tangles, synaptic and neuronal loss, neurotransmitter deficits and dementia) are indistinguishable when FAD is compared with typical, common, 'non-familial', or sporadic, AD (SAD). Studies of some clinically relevant mutant APP molecules from FAD families have yielded evidence that APP mutations can lead to the enhanced generation or aggregability of Abeta, consistent with a pathogenic role in AD. Other genetic loci for FAD have been discovered which are distinct from the immediate regulatory and coding regions of the APP gene, indicating that defects in molecules other than APP can also specify cerebral amyloidogenesis and FAD. To date, all APP and non-APP FAD mutations can be demonstrated to have the common feature of promoting amyloidogenesis of Abeta. Epidemiological studies indicate that postmenopausal women on estrogen replacement therapy (ERT) have their relative risk of developing SAD diminished by about one third as compared with age-matched women not receiving ERT [M.X. Tang, D. Jacobs, Y. Stern, K. Marder, P. Schofield, B. Gurland, H. Andrews, R. Mayeux, Effect of estrogen during menopause on risk and age at onset of Alzheimer's disease, Lancet 348 (2000) 429432]. Because of the key role of cerebral Abeta accumulation in initiating AD pathology, it is most attractive that estradiol might modulate SAD risk or age-at-onset by inhibiting Abeta accumulation. A possible mechanistic basis for such a scenario is reviewed here.

Amyloid precursor protein and membrane phospholipids in primary cortical neurons increase with development, or after exposure to nerve growth factor or Abeta(1-40)

We examined the relationships between membrane phospholipid levels, the secretion and expression of the amyloid precursor protein (APP), and the responses of both to nerve growth factor (NGF), Abeta(1-40) or Abeta(40-1) in developing cortical neurons cultured from rat embryos. Neuronal membrane phospholipid levels per cell, and phosphatidylcholine, phosphatidylserine, phosphatidylinositol and phosphatidylethanolamine increased individually between the first and seventh days of culturing. The amounts of APP holoprotein and APP mRNAs in the cells, as well as the amounts of soluble APP (APPs) secreted by them, also increased during neuronal development in vitro. The increases in APPs exceeded the increases in APP which, in turn, exceed those in phospholipid levels. The levels of APP holoprotein, but not of phospholipids, increased when neurons were grown in a choline-free medium, suggesting that increases in APP are not sufficient to stimulate changes in membrane phospholipids. Treatment of neuron cultures for four days with NGF or Abeta(1-40), but not with Abeta(40-1), dose-dependently increased membrane phospholipids, tau and GAP-43, as well as APP holoprotein and secreted APPs. These results indicate that agents, like NGF or Abeta(1-40), which enhance membrane phospholipid levels may promote neurite formation, APP expression and APPs secretion in primary neuronal cultures.

Analysis of the 5'-flanking region of the beta-amyloid precursor protein gene that contributes to increased promoter activity in differentiated neuronal cells

To study the transcription control of the beta-amyloid precursor protein (betaAPP) in Alzheimer's disease (AD), we functionally characterized the betaAPP gene promoter in differentiated cells. PC12 cells were first differentiated with nerve growth factor (NGF) and then transient transfection analysis was done with a series of 5'-deletion constructs, that extended as far upstream as -7900 down to +104 base pair (bp) relative to the transcription start site (+1). The truncated regions of the promoter were linked upstream to a reporter gene, chloramphenicol acetyl transferase (CAT). The CAT assay was performed to compare promoter activity of different 5'-flanking and intronic regions of the betaAPP gene. Our results suggest that the longest (-7900/+104) and one of the shortest (-47/+104) regions possessed significantly higher levels of promoter activity than the promoterless vector in NGF-differentiated PC12 cells. A deletion of about 7600 bp region from the -7900 to +104 construct resulted in 50% loss of original promoter activity. A deletion of all but 47 bp from the -7900 to +104 construct resulted in the loss of 66% (and retention of 34%) promoter activity. The region -3416/+104 bp displayed the strongest promoter activity whereas +1/+104 bp showed the least activity among all deletion constructs studied. The upstream region -5529 to -3416 contains a negative regulatory element and -3416 to -1131 contains a positive regulatory element. The very upstream region, -7900 to -3411, lacks independent functional activity. The 5'-UTR region (+1 to +104) showed minimum activity and the -75 to +104 region constitutes the basic promoter element. The first exon or a large part of the first intron (+99 to +6200) did not display any significant promoter activity. Thus, several positive and negative regulatory elements influence the basal level of betaAPP promoter activity in NGF-differentiated PC12 cells. We speculate that any structural alteration(s) due to a specific mutation in these regulatory regions can potentially alter the transcriptional machinery, and that can perhaps affect the level of beta-amyloid protein involved in AD.

Adenosine A(2A) receptor mRNA regulation by nerve growth factor is TrkA-, Src-, and Ras-dependent via extracellular regulated kinase and stress-activated protein kinase/c-Jun NH(2)-terminal kinase

We have shown previously that nerve growth factor (NGF) down-regulates adenosine A(2A) receptor (A(2A)AR) mRNA in PC12 cells. To define cellular mechanisms that modulate A(2A)AR expression, A(2A)AR mRNA and protein levels were examined in three PC12 sublines: i) PC12nnr5 cells, which lack the high affinity NGF receptor TrkA, ii) srcDN2 cells, which overexpress kinase-defective Src, and iii) 17.26 cells, which overexpress a dominant-inhibitory Ras. In the absence of functional TrkA, Src, or Ras, NGF-induced down-regulation of A(2A)AR mRNA and protein was significantly impaired. However, regulation of A(2A)AR expression was reconstituted in PC12nnr5 cells stably transfected with TrkA. Whereas NGF stimulated the mitogen-activated protein kinases p38, extracellular regulated kinase 1 and 2 (ERK1/ERK2), and stress-activated protein kinase/c-Jun NH(2)-terminal kinase (SAPK/JNK) in PC12 cells, these kinases were activated only partially or not at all in srcDN2 and 17.26 cells. Inhibiting ERK1/ERK2 with PD98059 or inhibiting SAPK/JNK by transfecting cells with a dominant-negative SAPKbeta/JNK3 mutant partially blocked NGF-induced down-regulation of A(2A)AR expression in PC12 cells. In contrast, inhibiting p38 with SB203580 had no effect on the regulation of A(2A)AR mRNA and protein levels. Treating SAPKbeta/JNK3 mutant-transfected PC12 cells with PD98059 completely abolished the NGF-induced decrease in A(2A)AR mRNA and protein levels. These results reveal a role for ERK1/ERK2 and SAPK/JNK in regulating A(2A)AR expression.

Neurohormonal Signaling Pathways and the Regulation of Alzheimer beta-Amyloid Precursor Metabolism

Postmenopausal women on estrogen replacement appear to have their relative risk of developing Alzheimer's disease diminished by about one half. Because brain amyloid accumulation plays a key role in initiating Alzheimer's pathology, it is attractive to postulate that estrogen might modulate Alzheimer's risk by inhibiting amyloid accumulation. Data and cell biological models supporting such a scenario are reviewed here.

Regulated secretion of amyloid precursor protein by TrkA receptor stimulation in rat pheochromocytoma-12 cells is mitogen activated protein kinase sensitive

We have shown recently in the pheochromocytoma PC-12 cell line, that the activation of the high-affinity receptor for nerve growth factor (NGF), tyrosine kinase receptor (TrkA), results in increased secretion of the amyloid precursor protein (APP) into the culture medium. In order to reveal through which TrkA-associated signaling pathway the secretory APP processing is mediated, signaling cascades activated by TrkA stimulation were selectively inhibited under conditions of selective TrkA stimulation via non-NGF mechanisms and APP secretion into the culture medium was followed by Western analysis. Our data demonstrate, that activation of mitogen activated protein (MAP) kinase alone is sufficient to promote APP secretion, whereas inhibition of MAP kinase will reduce APP secretion only when phospholipase Cgamma or phosphatidylinositol 3-kinase are additionally inhibited. This suggests that pharmacological manipulations activating the MAP kinase pathway may result in increased secretory APP processing.