Differential APP gene expression in rat cerebral cortex, meninges, and primary astroglial, microglial and neuronal cultures

Amyloid precursor protein induces reactive astrogliosis

AIM

Astrocytes respond to stressors by acquiring a reactive state characterized by changes in their morphology and function. Molecules underlying reactive astrogliosis, however, remain largely unknown. Given that several studies observed increase in the Amyloid Precursor Protein (APP) in reactive astrocytes, we here test whether APP plays a role in reactive astrogliosis.

METHODS

We investigated whether APP instigates reactive astroglios by examining in vitro and in vivo the morphology and function of naive and APP-deficient astrocytes in response to APP and well-established stressors.

RESULTS

Overexpression of APP in cultured astrocytes led to remodeling of the intermediate filament network, enhancement of cytokine production, and activation of cellular programs centered around the interferon (IFN) pathway, all signs of reactive astrogliosis. Conversely, APP deletion abrogated remodeling of the intermediate filament network and blunted expression of IFN-stimulated gene products in response to lipopolysaccharide. Following traumatic brain injury (TBI), mouse reactive astrocytes also exhibited an association between APP and IFN, while APP deletion curbed the increase in glial fibrillary acidic protein observed canonically in astrocytes in response to TBI.

CONCLUSIONS

The APP thus represents a candidate molecular inducer and regulator of reactive astrogliosis. This finding has implications for understanding pathophysiology of neurodegenerative and other diseases of the nervous system characterized by reactive astrogliosis and opens potential new therapeutic avenues targeting APP and its pathways to modulate reactive astrogliosis.

Normal-Tension Glaucoma and Potential Clinical Links to Alzheimer's Disease

Glaucoma is a group of optic neuropathies and the world's leading cause of irreversible blindness. Normal-tension glaucoma (NTG) is a subtype of glaucoma that is characterized by a typical pattern of peripheral retinal loss, in which the patient's intraocular pressure (IOP) is considered within the normal range (<21 mmHg). Currently, the only targetable risk factor for glaucoma is lowering IOP, and patients with NTG continue to experience visual field loss after IOP-lowering treatments. This demonstrates the need for a better understanding of the pathogenesis of NTG and underlying mechanisms leading to neurodegeneration. Recent studies have found significant connections between NTG and cerebral manifestations, suggesting NTG as a neurodegenerative disease beyond the eye. Gaining a better understanding of NTG can potentially provide new Alzheimer's Disease diagnostics capabilities. This review identifies the epidemiology, current biomarkers, altered fluid dynamics, and cerebral and ocular manifestations to examine connections and discrepancies between the mechanisms of NTG and Alzheimer's Disease.

Amyloid precursor protein induces reactive astrogliosis

We present in vitro and in vivo evidence demonstrating that Amyloid Precursor Protein (APP) acts as an essential instigator of reactive astrogliosis. Cell-specific overexpression of APP in cultured astrocytes led to remodelling of the intermediate filament network, enhancement of cytokine production and activation of cellular programs centred around the interferon (IFN) pathway, all signs of reactive astrogliosis. Conversely, APP deletion in cultured astrocytes abrogated remodelling of the intermediate filament network and blunted expression of IFN stimulated gene (ISG) products in response to lipopolysaccharide (LPS). Following traumatic brain injury (TBI), mouse reactive astrocytes also exhibited an association between APP and IFN, while APP deletion curbed the increase in glial fibrillary acidic protein (GFAP) observed canonically in astrocytes in response to TBI. Thus, APP represents a molecular inducer and regulator of reactive astrogliosis.

Physiological Roles of β-amyloid in Regulating Synaptic Function: Implications for AD Pathophysiology

The physiological functions of endogenous amyloid-β (Aβ), which plays important role in the pathology of Alzheimer's disease (AD), have not been paid enough attention. Here, we review the multiple physiological effects of Aβ, particularly in regulating synaptic transmission, and the possible mechanisms, in order to decipher the real characters of Aβ under both physiological and pathological conditions. Some worthy studies have shown that the deprivation of endogenous Aβ gives rise to synaptic dysfunction and cognitive deficiency, while the moderate elevation of this peptide enhances long term potentiation and leads to neuronal hyperexcitability. In this review, we provide a new view for understanding the role of Aβ in AD pathophysiology from the perspective of physiological meaning.

Knockdown of Amyloid Precursor Protein: Biological Consequences and Clinical Opportunities

Amyloid precursor protein (APP) and its cleavage fragment Amyloid-β (Aβ) have fundamental roles in Alzheimer's disease (AD). Genetic alterations that either increase the overall dosage of APP or alter its processing to favour the generation of longer, more aggregation prone Aβ species, are directly causative of the disease. People living with one copy of APP are asymptomatic and reducing APP has been shown to lower the relative production of aggregation-prone Aβ species in vitro. For these reasons, reducing APP expression is an attractive approach for AD treatment and prevention. In this review, we will describe the structure and the known functions of APP and go on to discuss the biological consequences of APP knockdown and knockout in model systems. We highlight progress in therapeutic strategies to reverse AD pathology via reducing APP expression. We conclude that new technologies that reduce the dosage of APP expression may allow disease modification and slow clinical progression, delaying or even preventing onset.

Human microRNA (miR-20b-5p) modulates Alzheimer's disease pathways and neuronal function, and a specific polymorphism close to the MIR20B gene influences Alzheimer's biomarkers

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with loss of cognitive, executive, and other mental functions, and is the most common form of age-related dementia. Amyloid-β peptide (Aβ) contributes to the etiology and progression of the disease. Aβ is derived from the amyloid-β precursor protein (APP). Multiple microRNA (miRNA) species are also implicated in AD. We report that human hsa-miR20b-5p (miR-20b), produced from the MIR20B gene on Chromosome X, may play complex roles in AD pathogenesis, including Aβ regulation. Specifically, miR-20b-5p miRNA levels were altered in association with disease progression in three regions of the human brain: temporal neocortex, cerebellum, and posterior cingulate cortex. In cultured human neuronal cells, miR-20b-5p treatment interfered with calcium homeostasis, neurite outgrowth, and branchpoints. A single-nucleotide polymorphism (SNP) upstream of the MIR20B gene (rs13897515) associated with differences in levels of cerebrospinal fluid (CSF) Aβ1-42 and thickness of the entorhinal cortex. We located a miR-20b-5p binding site in the APP mRNA 3'-untranslated region (UTR), and treatment with miR-20b-5p reduced APP mRNA and protein levels. Network analysis of protein-protein interactions and gene coexpression revealed other important potential miR-20b-5p targets among AD-related proteins/genes. MiR-20b-5p, a miRNA that downregulated APP, was paradoxically associated with an increased risk for AD. However, miR-20b-5p also reduced, and the blockade of APP by siRNA likewise reduced calcium influx. As APP plays vital roles in neuronal health and does not exist solely to be the source of "pathogenic" Aβ, the molecular etiology of AD is likely to not just be a disease of "excess" but a disruption of delicate homeostasis.

Not just amyloid: physiological functions of the amyloid precursor protein family

Amyloid precursor protein (APP) gives rise to the amyloid-β peptide and thus has a key role in the pathogenesis of Alzheimer disease. By contrast, the physiological functions of APP and the closely related APP-like proteins (APLPs) remain less well understood. Studying these physiological functions has been challenging and has required a careful long-term strategy, including the analysis of different App-knockout and Aplp-knockout mice. In this Review, we summarize these findings, focusing on the in vivo roles of APP family members and their processing products for CNS development, synapse formation and function, brain injury and neuroprotection, as well as ageing. In addition, we discuss the implications of APP physiology for therapeutic approaches.

Region-Specific Differences in Amyloid Precursor Protein Expression in the Mouse Hippocampus

The physiological role of amyloid precursor protein (APP) has been extensively investigated in the rodent hippocampus. Evidence suggests that APP plays a role in synaptic plasticity, dendritic and spine morphogenesis, neuroprotection and—at the behavioral level—hippocampus-dependent forms of learning and memory. Intriguingly, however, studies focusing on the role of APP in synaptic plasticity have reported diverging results and considerable differences in effect size between the dentate gyrus (DG) and area CA1 of the mouse hippocampus. We speculated that regional differences in APP expression could underlie these discrepancies and studied the expression of APP in both regions using immunostaining, in situ hybridization (ISH), and laser microdissection (LMD) in combination with quantitative reverse transcription polymerase chain reaction (RT-qPCR) and western blotting. In sum, our results show that APP is approximately 1.7-fold higher expressed in pyramidal cells of Ammon’s horn than in granule cells of the DG. This regional difference in APP expression may explain why loss-of-function approaches using APP-deficient mice revealed a role for APP in Hebbian plasticity in area CA1, whereas this could not be shown in the DG of the same APP mutants.

Glutamate system, amyloid ß peptides and tau protein: functional interrelationships and relevance to Alzheimer disease pathology

Alzheimer disease is the most prevalent form of dementia globally and is characterized premortem by a gradual memory loss and deterioration of higher cognitive functions and postmortem by neuritic plaques containing amyloid ß peptide and neurofibrillary tangles containing phospho-tau protein. Glutamate is the most abundant neurotransmitter in the brain and is essential to memory formation through processes such as long-term potentiation and so might be pivotal to Alzheimer disease progression. This review discusses how the glutamatergic system is impaired in Alzheimer disease and how interactions of amyloid ß and glutamate influence synaptic function, tau phosphorylation and neurodegeneration. Interestingly, glutamate not only influences amyloid ß production, but also amyloid ß can alter the levels of glutamate at the synapse, indicating that small changes in the concentrations of both molecules could influence Alzheimer disease progression. Finally, we describe how the glutamate receptor antagonist, memantine, has been used in the treatment of individuals with Alzheimer disease and discuss its effectiveness.

Amyloid precursor protein and proinflammatory changes are regulated in brain and adipose tissue in a murine model of high fat diet-induced obesity

Background

Middle age obesity is recognized as a risk factor for Alzheimer's disease (AD) although a mechanistic linkage remains unclear. Based upon the fact that obese adipose tissue and AD brains are both areas of proinflammatory change, a possible common event is chronic inflammation. Since an autosomal dominant form of AD is associated with mutations in the gene coding for the ubiquitously expressed transmembrane protein, amyloid precursor protein (APP) and recent evidence demonstrates increased APP levels in adipose tissue during obesity it is feasible that APP serves some function in both disease conditions.

Methodology/Principal Findings

To determine whether diet-induced obesity produced proinflammatory changes and altered APP expression in brain versus adipose tissue, 6 week old C57BL6/J mice were maintained on a control or high fat diet for 22 weeks. Protein levels and cell-specific APP expression along with markers of inflammation and immune cell activation were compared between hippocampus, abdominal subcutaneous fat and visceral pericardial fat. APP stimulation-dependent changes in macrophage and adipocyte culture phenotype were examined for comparison to the in vivo changes.

Conclusions/Significance

Adipose tissue and brain from high fat diet fed animals demonstrated increased TNF-α and microglial and macrophage activation. Both brains and adipose tissue also had elevated APP levels localizing to neurons and macrophage/adipocytes, respectively. APP agonist antibody stimulation of macrophage cultures increased specific cytokine secretion with no obvious effects on adipocyte culture phenotype. These data support the hypothesis that high fat diet-dependent obesity results in concomitant pro-inflammatory changes in brain and adipose tissue that is characterized, in part, by increased levels of APP that may be contributing specifically to inflammatory changes that occur.

Amyloid precursor protein expression modulates intestine immune phenotype

Amyloid precursor protein (APP) is widely expressed across many tissue and cell types. Proteolytic processing of the protein gives rise to a plethora of protein fragments with varied biological activities. Although a large amount of data has been generated describing the metabolism of the protein in neurons, its role in regulating the phenotype of other cells remains unclear. Based upon prior work demonstrating that APP regulates the activation phenotype of monocytic lineage cells, we hypothesized that APP can regulate macrophage activation phenotype in tissues other than brain. Ileums of the small intestines from C57BL6/J wild type and APP(-/-) mice were compared as a representative tissue normally associated with abundant macrophage infiltration. APP(-/-) intestines demonstrated diminished CD68 immunoreactivity compared to wild type mice. This correlated with significantly less cyclooxygenase-2 (cox-2), CD68, CD40, CD11c, and βIII-tubulin protein levels. Peritoneal macrophages from APP(-/-) mice demonstrated decreased in vitro migratory ability compared to wild type cells and diminished basal KC cytokine secretion. Whereas, APP(-/-) intestinal macrophages had an increase in basal KC cytokine secretion compared to wild type cells. Conversely, there was a significant decrease in multiple cytokine levels in APP(-/-) compared to wild type ileums. Finally, APP(-/-) mice demonstrated impaired absorption and increased motility compared to wild type mice. These data demonstrate the APP expression regulates immune cell secretions and phenotype and intestinal function. This data set describes a novel function for this protein or its metabolites that may be relevant not only for Alzheimer's disease but a range of immune-related disorders.

Activation of extrasynaptic, but not synaptic, NMDA receptors modifies amyloid precursor protein expression pattern and increases amyloid-ß production

Calcium is a key mediator controlling essential neuronal functions depending on electrical activity. Altered neuronal calcium homeostasis affects metabolism of amyloid precursor protein (APP), leading to increased production of β-amyloid (Aβ), and contributing to the initiation of Alzheimer's disease (AD). A linkage between excessive glutamate receptor activation and neuronal Aβ release was established, and recent reports suggest that synaptic and extrasynaptic NMDA receptor (NMDAR) activation may have distinct consequences in plasticity, gene regulation, and neuronal death. Here, we report for the first time that prolonged activation of extrasynaptic NMDAR, but not synaptic NMDAR, dramatically increased the neuronal production of Aβ. This effect was preceded by a shift from APP695 to Kunitz protease inhibitory domain (KPI) containing APPs (KPI-APPs), isoforms exhibiting an important amyloidogenic potential. Conversely, after synaptic NMDAR activation, we failed to detect any KPI-APP expression and neuronal Aβ production was not modified. Calcium imaging data showed that intracellular calcium concentration after extrasynaptic NMDAR stimulation was lower than after synaptic activation. This suggests distinct signaling pathways for each pool of receptors. We found that modification of neuronal APP expression pattern triggered by extrasynaptic NMDAR activation was regulated at an alternative splicing level involving calcium-/calmodulin-dependent protein kinase IV, but overall APP expression remained identical. Finally, memantine dose-dependently inhibited extrasynaptic NMDAR-induced KPI-APPs expression as well as neuronal Aβ release. Altogether, these data suggest that a chronic activation of extrasynaptic NMDAR promotes amyloidogenic KPI-APP expression leading to neuronal Aβ release, representing a causal risk factor for developing AD.

Interleukin-1beta enhances nucleotide-induced and alpha-secretase-dependent amyloid precursor protein processing in rat primary cortical neurons via up-regulation of the P2Y(2) receptor

The heterologous expression and activation of the human P2Y(2) nucleotide receptor (P2Y(2)R) in human 1321N1 astrocytoma cells stimulates alpha-secretase-dependent cleavage of the amyloid precursor protein (APP), causing extracellular release of the non-amyloidogenic protein secreted amyloid precursor protein (sAPPalpha). To determine whether a similar response occurs in a neuronal cell, we analyzed whether P2Y(2)R-mediated production of sAPPalpha occurs in rat primary cortical neurons (rPCNs). In rPCNs, P2Y(2)R mRNA and receptor activity were virtually absent in quiescent cells, whereas overnight treatment with the pro-inflammatory cytokine interleukin-1beta (IL-1beta) up-regulated both P2Y(2)R mRNA expression and receptor activity by four-fold. The up-regulation of the P2Y(2)R was abrogated by pre-incubation with Bay 11-7085, an IkappaB-alpha phosphorylation inhibitor, which suggests that P2Y(2)R mRNA transcript levels are regulated through nuclear factor-kappa-B (NFkappaB) signaling. Furthermore, the P2Y(2)R agonist Uridine-5'-triphosphate (UTP) enhanced the release of sAPPalpha in rPCNs treated with IL-1beta or transfected with P2Y(2)R cDNA. UTP-induced release of sAPPalpha from rPCNs was completely inhibited by pre-treatment of the cells with the metalloproteinase inhibitor TACE inhibitor (TAPI-2) or the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002, and was partially inhibited by the MAPK/extracellular signal-regulated kinase inhibitor U0126 and the protein kinase C inhibitor GF109203. These data suggest that P2Y(2)R-mediated release of sAPPalpha from cortical neurons is directly dependent on a disintegrin and metalloproteinase (ADAM) 10/17 and PI3K activity, whereas extracellular signal-regulated kinase 1/2 and PI3K activity may indirectly regulate APP processing. These results demonstrate that elevated levels of pro-inflammatory cytokines associated with neurodegenerative diseases, such as IL-1beta, can enhance non-amyloidogenic APP processing through up-regulation of the P2Y(2)R in neurons.

APP processing and the APP-KPI domain involvement in the amyloid cascade

Alternative APP mRNA splicing can generate isoforms of APP containing a Kunitz protease inhibitor (KPI) domain. KPI is one of the main serine protease inhibitors. Protein and mRNA KPI(+)APP levels are elevated in Alzheimer's disease (AD) brain and are associated with increased amyloid beta deposition. In the last years increasing evidence on multiple points in the amyloid cascade where KPI(+)APP is involved has been accumulated, admitting an outstanding position in the pathogenesis of AD to the KPI domain. This review focuses on the APP processing, the molecular activity of KPI and its physiological and pathological roles and the KPI involvement in the amyloid cascade through the nerve growth factor, the lipoprotein receptor-related protein, the tumor necrosis factor-alpha converting enzyme and the Notch1 protein.

NMDA receptor activation inhibits alpha-secretase and promotes neuronal amyloid-beta production

Acute brain injuries have been identified as a risk factor for developing Alzheimer's disease (AD). Because glutamate plays a pivotal role in these pathologies, we studied the influence of glutamate receptor activation on amyloid-beta (Abeta) production in primary cultures of cortical neurons. We found that sublethal NMDA receptor activation increased the production and secretion of Abeta. This effect was preceded by an increased expression of neuronal Kunitz protease inhibitory domain (KPI) containing amyloid-beta precursor protein (KPI-APP) followed by a shift from alpha-secretase to beta-secretase-mediated APP processing. This shift is a result of the inhibition of the alpha-secretase candidate tumor necrosis factor-alpha converting enzyme (TACE) when associated with neuronal KPI-APPs. This KPI-APP/TACE interaction was also present in AD brains. Thus, our findings reveal a cellular mechanism linking NMDA receptor activation to neuronal Abeta secretion. These results suggest that even mild deregulation of the glutamatergic neurotransmission may increase Abeta production and represent a causal risk factor for developing AD.

Amyloid precursor protein expression in the rat hippocampal dentate gyrus modulates during memory consolidation

Despite advances in our understanding of the basic biology of amyloid precursor protein (APP), the normal physiological function(s) of APP in learning and memory remains unclear. Here we show increased APP degradation in the hippocampus to be associated with the consolidation of a passive avoidance response. Neurone-specific APP695 expression became transiently reduced 2-4 h post-training through association with endosomal adaptin proteins and enhanced internalization. By contrast, internalization of glial-associated APP containing a Kunitz protease inhibitor-like domain (APP-KPI) was dependent on the low-density lipoprotein receptor-related protein (LRP). In addition, LRP expression and association with apolipoprotein E increased in the 2-4 h post-training period. The LRP antagonist receptor-associated protein prevented the APP-KPI internalization and LRP-apolipoprotein E association and this resulted in amnesia. Degradation of APP695 and APP-KPI did not appear to be related to alpha-secretase activity, as no learning-associated increase of secreted APP was observed in the CSF. Moreover, as internalization of APP isoforms was observed only in dentate gyrus, it probably relates to the learning-associated restructuring of the perforant path terminals. Memory-associated APP processing in both neuronal and glial compartments points to a role for glial unsheathing of synaptic connections, an event required for the synaptic restructuring that accompanies memory consolidation. These observations may have a direct relevance to understanding the pathophysiology of Alzheimer's disease as beta/gamma-secretase-derived beta-amyloid is formed following internalization of cell surface APP into the endosomal compartment.

Transforming growth factor-beta 1 potentiates amyloid-beta generation in astrocytes and in transgenic mice

Accumulation of the amyloid-beta peptide (Abeta) in the brain is crucial for development of Alzheimer's disease. Expression of transforming growth factor-beta1 (TGF-beta1), an immunosuppressive cytokine, has been correlated in vivo with Abeta accumulation in transgenic mice and recently with Abeta clearance by activated microglia. Here, we demonstrate that TGF-beta1 drives the production of Abeta40/42 by astrocytes leading to Abeta production in TGF-beta1 transgenic mice. First, TGF-beta1 induces the overexpression of the amyloid precursor protein (APP) in astrocytes but not in neurons, involving a highly conserved TGF-beta1-responsive element in the 5'-untranslated region (+54/+74) of the APP promoter. Second, we demonstrated an increased release of soluble APP-beta which led to TGF-beta1-induced Abeta generation in both murine and human astrocytes. These results demonstrate that TGF-beta1 potentiates Abeta production in human astrocytes and may enhance the formation of plaques burden in the brain of Alzheimer's disease patients.

Regulation of beta-amyloid precursor protein and inositol 1,4,5-trisphosphate receptor gene expression during differentiation of a human neuronal cell line

Retinoic acid-induced differentiation of SH-SY5Y human neuroblastoma cells results in the development of extensive neurite processes as well as changes in cell body morphology toward a neuronal phenotype. The authors have examined concurrent regulation of beta-amyloid precursor protein (APP) and inositol 1,4,5-trisphosphate receptor (insP(3)R) gene expression in SY5Y cells during neuronal differentiation. Of the multiple APP mRNA transcripts expressed in this cell line, retinoic acid treatment significantly increased the expression of APP(695) transcript while the level of total APP remained unchanged. In the same time course, neuronal differentiation decreased the expression of insP(3)R at both the mRNA and protein levels. These findings demonstrate an inverse relationship between APP and insP(3)R gene expression during neuronal differentiation of SH-SY5Y cells and suggest a possible change in intracellular calcium homeostasis.

Signal transduction through tyrosine-phosphorylated C-terminal fragments of amyloid precursor protein via an enhanced interaction with Shc/Grb2 adaptor proteins in reactive astrocytes of Alzheimer's disease brain

The proteolytic processing of amyloid precursor protein (APP) through the formation of membrane-bound C-terminal fragments (CTFs) and of soluble beta-amyloid peptides likely influences the development of Alzheimer's disease (AD). We show that in human brain a subset of CTFs are tyrosine-phosphorylated and form stable complexes with the adaptor protein ShcA. Grb2 is also part of these complexes, which are present in higher amounts in AD than in control brains. ShcA immunoreactivity is also greatly enhanced in patients with AD and occurs at reactive astrocytes surrounding cerebral vessels and amyloid plaques. A higher amount of phospho-ERK1,2, likely as result of the ShcA activation, is present in AD brains. In vitro experiments show that the ShcA-CTFs interaction is strictly confined to glial cells when treated with thrombin, which is a well known ShcA and ERK1,2 activator and a regulator of APP cleavage. In untreated cells ShcA does not interact with either APP or CTFs, although they are normally generated. Altogether these data suggest that CTFs are implicated in cell signaling via Shc transduction machinery, likely influencing MAPK activity and glial reaction in AD patients.

Evidence for binding of the ectodomain of amyloid precursor protein 695 and activated high molecular weight kininogen

To identify ligands that bind to the N-terminal portion of human amyloid precursor protein (APP), we sought binding partners for a fragment of the ectodomain of human APP695 (sAPP(695)T). The probe bound to fragments of high molecular weight kininogen (HK) in rat cortical membrane preparations in vitro. Laser confocal microscopy indicated that APP and HK colocalize near cerebral blood vessels, in the neuropil, and in many neurons of rat brain. sAPP(695)T bound to human activated kininogen (HKa) (K(d)=0.3+/-0.1 nM), but not to inactivated or low molecular weight kininogen. Binding was specific for the light chain sequence of HKa. Biotinylated human HKa also bound to sAPP(695) (K(d)=0.3+/-0.5 nM). sAPP(695) and HKa form tight complexes in solution that can be coimmunoprecipitated. These results support the hypothesis that forms of APP and kininogen can interact in brain tissue. Considering the implications of APP in neurite outgrowth, the APP-HKa interaction could modulate neurogenesis.

Astrocytes down-regulate neuronal beta-amyloid precursor protein expression and modify its processing in an apolipoprotein E isoform-specific manner

Alzheimer's disease is the most frequent neurodegenerative disorder in the aged population and is characterized by the deposition of the 40/42-residue amyloid beta protein (A beta), a proteolytic fragment of the beta-amyloid precursor protein (APP). A common apolipoprotein E (apoE) polymorphism is associated with an increased risk of developing the disease. In order to assess the putative relationship between apoE and amyloidogenesis in the CNS, we prepared primary cortical neurons overexpressing humanized APP695 bearing the Swedish mutation (hAPP(695sw)) and we analysed APP expression and processing after: (i) coculture with primary astrocytes from wild-type, apoE-deficient (E0) mice, or mice overexpressing human apoE2, E3, or E4; (ii) treatment with conditioned media from apoE0, E2, E3 or E4 astrocytes; and (iii) treatment with human recombinant ApoE or human apoE purified from conditioned media of stably transfected RAW264 cells (E2, E3 and E4). Interestingly, a strong decrease in APP expression was observed only when neurons were cocultured with astrocytes (and independently of the apoE genotype considered), suggesting that cell-cell contact is required. Moreover, apoE4-secreting astrocytes, but not recombinant or purified apoE4, significantly increased A beta production and decrease sAPP alpha secretion only when cultured in direct contact with neurons, whereas apoE2 astrocytes had a protective effect. We conclude that astrocytes: (i) strongly regulate neuronal APP expression in primary neurons, and (ii) promote the amyloidogenic pathway in an apoE4-dependent manner. Thus, apoE and astrocytic factor(s) may modulate the pathogenesis of Alzheimer's disease.

The amyloid-beta peptide and its role in Alzheimer's disease

Amyloid formation plays a central role in the cause and progression of Alzheimer's disease. The major component of this amyloid is the amyloid-beta (A beta) peptide, which is currently the subject of intense study. This review discusses some recent studies in the area of A beta synthesis, purification and structural analysis. Also discussed are proposed mechanisms for A beta-induced neurotoxicity and some recent advances in the development of A beta-related therapeutic strategies.

Effect of estradiol on neuronal Swedish-mutated beta-amyloid precursor protein metabolism: reversal by astrocytic cells

Alzheimer's disease is the most frequent neurodegenerative disorder in the aged population and is characterized by the deposition of the 40/42-residue amyloid beta protein (Abeta), a proteolytic fragment of the beta-amyloid precursor protein (APP). Recently, it has been shown that physiological doses of estradiol reduce the generation of endogenous Abeta in primary cortical neurons. Here we investigate the influence of estrogen in amyloidogenesis and sAPPalpha secretion in the CNS. By means of primary cortical neurons overexpressing humanized APP(695) bearing the Swedish mutation (hAPP(695sw)), we analyzed APP maturation in the absence or in the presence of estrogen. We show that estrogen at a 2 microM concentration increases the release of the neuroprotective sAPPalpha fragment but does not reduce the release of Abeta in primary neurons overexpressing the Swedish-mutated form of APP. Furthermore, neurons cocultured with astrocytic cells or grown with astrocytes conditioned media do not exhibit the estrogen-induced increase in sAPPalpha secretion. Altogether, our data indicate that astrocytes interfere with estrogen in the regulation of sAPPalpha secretion, probably via secreted factor(s).

Guinea-pig primary cell cultures provide a model to study expression and amyloidogenic processing of endogenous amyloid precursor protein

Until now guinea-pigs have been rarely used to investigate formation and deposition of Alzheimer's disease-associated amyloid beta peptides despite the sequence identity of human and guinea-pig amyloid beta peptides being known, and the overall similarity of human and guinea-pig amyloid precursor protein. We now describe a primary cell culture system of mixed fetal guinea-pig brain cells, which we have applied to characterize endogenous amyloid precursor protein processing and amyloid beta formation. These cell cultures were established at embryonic day 24 of guinea-pigs after comparison of selected stages of guinea-pig ontogenetic development with the known ontogeny of rats, and were characterized by immunocytochemical detection of neuronal and glial marker proteins. Amyloid precursor protein expression, processing and amyloid beta formation increased in parallel with cellular maturation during cultivation and reached a stable phase after approximately 14 days in vitro therefore providing a suitable time for analysis. Aged cultures display strong neuronal amyloid precursor protein immunoreactivity and an altered profile of amyloid precursor protein isoform messenger RNA expression due to glial proliferation as single neurons were shown to retain their typical pattern of amyloid precursor protein expression. We show that amyloid precursor protein in guinea-pig cells is processed by different protease activities which most likely represent alpha- and beta-secretase, leading to the generation of soluble amyloid precursor protein derivatives. Furthermore, endogenous amyloid precursor protein processing leads to production of substantial amounts of amyloid beta-peptides which accumulate in conditioned culture medium. Amyloid beta was readily detectable by western blot analysis and was shown to consist of approximately 80-90% amyloid beta(1-40). We suggest that primary guinea-pig cell cultures provide a valuable tool in amyloid research that resembles amyloid precursor protein processing under physiological concentrations and, therefore, the situation in humans more closely than current rodent models. It should be especially useful in screening experiments for secretase inhibiting compounds.

Uptake, degradation, and release of fibrillar and soluble forms of Alzheimer's amyloid beta-peptide by microglial cells

Microglia are phagocytic cells that are the main inflammatory response cells of the central nervous system. In Alzheimer's disease brain, activated microglia are concentrated in regions of compact amyloid deposits that contain the 39-43-amino acid Abeta peptide. We examined the uptake, degradation, and release of small aggregates of fibrillar Abeta (fAbeta) or soluble Abeta (sAbeta) by microglia. We found that although some degradation of fAbeta was observed over 3 days, no further degradation was observed over the next 9 days. Instead, there was a slow release of intact Abeta. The poor degradation was not due to inhibition of lysosomal function, since the rate of alpha2-macroglobulin degradation was not affected by the presence of fAbeta in the late endosomes/lysosomes. In contrast to fAbeta, internalization of sAbeta was not saturable. After internalization, sAbeta was released rapidly from microglia, and very little was degraded. These data show that fAbeta and sAbeta interact differently with microglia but that after internalization a large fraction of both are released without degradation.

Regulation of Drosophila FMRFamide neuropeptide gene expression

Physiologically important peptides are often encoded in precursors that contain several gene products; thus, regulation of expression of polypeptide proteins is crucial to transduction pathways. Differential processing of precursors by cell- or tissue-specific proteolytic enzymes can yield messengers with diverse distributions and dissimilar activities. FMRFamide-related peptides (FaRPs) are present throughout the animal kingdom and affect both neural and gastrointestinal functions. Organisms have several genes encoding numerous FaRPs with a common C-terminal structure but different N-terminal amino acid extensions. We have isolated SDNFMRFamide, DPKQDFMRFamide, and TPAEDFMRFamide contained in the Drosophila FMRFamide gene. To investigate the regulation of expression of FMRFamide peptides, we generated antisera to distinguish among the three neuropeptides. We have previously reported the distribution of SDNFMRFamide and DPKQDFMRFamide. In this article, we describe TPAEDFMRFamide expression. TPAEDFMRFamide antisera stain cells in embryonic, larval, pupal, and adult thoracic and abdominal ganglia. In addition, TPAEDFMRFamide-immunoreactive material is present in a lateral protocerebrum cell in adult. Thus, TPAEDFMRFamide antisera staining of neural tissue is different from SDNFMRFamide or DPKQDFMRFamide. In addition, TPAEDFMRFamide antisera stain larval, pupal, and adult gut, while SDNFMRFamide and DPKQDFMRFamide do not. TPAEDFMRFamide immunoreactivity is present in cells stained by FMRFamide antisera. Taken together, these data support the conclusion that TPAEDFMRFamide is differentially processed from the FMRFamide polypeptide protein precursor and may act in both neural and gastrointestinal tissue.

Differential processing of neuropeptides influences Drosophila heart rate

Peptides that play critical physiological roles are often encoded in precursors that contain several structurally-related gene products. Differential processing of a precursor by cell-specific processing enzymes can yield multiple messengers with diverse distributions and activities. We have reported the isolation of SDNFMRFamide, DPKQDFMRFamide, and TPAEDFMRFamide from adult Drosophila melanogaster. The peptides are encoded in the FMRFamide gene and have a common C-terminal FMRFamide but different N-terminal extensions. In order to investigate the processing of the FMRFamide polypeptide protein precursor, we generated antisera to distinguish among the structurally-related neuropeptides. Utilizing a triple-label immunofluorescent protocol, we mapped the distribution of the peptides. Each peptide has a unique, non-overlapping cellular expression pattern in neural tissue suggesting that the precursor is differentially processed. In order to identify a biological activity of the peptides, we established an in vivo heart rate assay. SDNFMRFamide decreases heart rate but DPKQDFMRFamide and TPAEDFMRFamide do not, indicating that the N-terminal residues are critical for this activity. SDNFMRFamide immunoreactivity is present in the aorta, implying that SDNFMRFamide acts locally to affect heart rate; DPKQDFMRFamide and TPAEDFMRFamide antisera do not stain cardiac tissue. Our data support the conclusion that Drosophila contains cell-specific proteolytic enzymes to differentially process a polypeptide protein precursor resulting in unique expression patterns of structurally-related, yet functionally distinct neuropeptides.

Amyloid precursor protein of Alzheimer's disease: evidence for a stable, full-length, trans-membrane pool in primary neuronal cultures

We and colleagues have shown that the amyloid protein precursor of Alzheimer's disease (APP) is distributed along the surface of neurites of fixed but nonpermeabilized neurons in primary culture in a segmental pattern, which shows colocalization with some markers of adhesion patches. This is in contrast to the diffuse pattern of immunoreactivity seen after permeabilization. We have also recently demonstrated that the APP in these surface patches is likely to be integral to the membrane rather than secreted and re-adsorbed, based on alkali stripping experiments and on soluble APP adsorption experiments. Total cellular APP has previously been shown to have a short half-life of approximately 30-60 min. We confirm this in neurons in primary culture in pulse-chase experiments using short labelling times. Additionally, we provide evidence that a separate, stable pool of neuronal APP can be demonstrated in pulse-chase experiments using long labelling times. Experiments involving inhibition of protein synthesis suggest that this corresponds with the surface, segmental pool. Metabolic labelling followed by surface biotinylation and two-stage precipitation demonstrates that the surface APP is trans-membrane and full-length (not carboxyl-terminal truncated), and confirms that the surface APP belongs to the stable pool. This two-stage procedure is necessary as the surface APP appears to be present in low copy number, and is difficult to detect by direct labelling. This information is consistent with a role for APP in stable cell-matrix or cell-cell interactions.

Translation of the alzheimer amyloid precursor protein mRNA is up-regulated by interleukin-1 through 5'-untranslated region sequences

The amyloid precursor protein (APP) has been associated with Alzheimer's disease (AD) because APP is processed into the beta-peptide that accumulates in amyloid plaques, and APP gene mutations can cause early onset AD. Inflammation is also associated with AD as exemplified by increased expression of interleukin-1 (IL-1) in microglia in affected areas of the AD brain. Here we demonstrate that IL-1alpha and IL-1beta increase APP synthesis by up to 6-fold in primary human astrocytes and by 15-fold in human astrocytoma cells without changing the steady-state levels of APP mRNA. A 90-nucleotide sequence in the APP gene 5'-untranslated region (5'-UTR) conferred translational regulation by IL-1alpha and IL-1beta to a chloramphenicol acetyltransferase (CAT) reporter gene. Steady-state levels of transfected APP(5'-UTR)/CAT mRNAs were unchanged, whereas both base-line and IL-1-dependent CAT protein synthesis were increased. This APP mRNA translational enhancer maps from +55 to +144 nucleotides from the 5'-cap site and is homologous to related translational control elements in the 5'-UTR of the light and and heavy ferritin genes. Enhanced translation of APP mRNA provides a mechanism by which IL-1 influences the pathogenesis of AD.

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.

Rat microglia exhibit increased density on Alzheimer's plaques in vitro

The relationship of microglia to senile plaques was investigated by culturing glial cells derived from neonatal rat brain on cryostat sections of Alzheimer's disease (AD) or control brain. Rat microglia were identified by their uptake of DiI-acetylated LDL. Plaques were colocalized using Thioflavin-S staining. Although the number of microglia attached to AD tissue sections did not differ significantly from the number on control brain tissue, the density of microglia on senile plaques was significantly greater than on nonsenile plaque areas of the same sections. These results suggest that microglia may have a higher affinity for senile plaques than for nonsenile plaque regions of AD brain tissue and are consistent with the hypothesis that microglia respond to plaques.

Cell-specific expression of beta-amyloid precursor protein isoform mRNAs and proteins in neurons and astrocytes

The abnormal accumulation of beta-amyloid (A beta) in senile plaques appears to be a central pathological process in Alzheimer's disease. A beta is formed by proteolysis of beta-amyloid precursor protein (APP) with several isoforms generated by alternative splicing of exons 7, 8 and 15. A semi-quantitative reverse transcription (RT)-polymerase chain reaction (PCR) analysis showed that APP695 mRNA lacking exon 7 and 8 was most abundant in primary cultures of rat neurons, while APP770 and APP751 representing, respectively, the full length and exon 8 lacking isoforms predominated in cultured astroglial cells. Antisera AP-2 and AP-4 were produced by immunizing rabbits with keyhole limpet haemocyanin coupled with synthetic peptides representing KPI region APP301-316 and A beta region APP670-686 of APP770, respectively. These polyclonal antisera were purified against the corresponding peptide using affinity chromatography. Western blot analysis of homogenates of relatively enriched neuronal and astroglial cultures showed that these antibodies discretely stained bands of proteins in a cell-specific manner. Dot-blot analysis using AP-2, AP-4 and 22C11 antibodies indicated that, in comparison with neurons, cultured astrocytes contained 3-fold greater KPI-containing APP isoform proteins. The amount of total APP proteins, which include both KPI-containing and KPI-lacking APP isoforms, was approximately 90% higher in astrocytes than in neurons. Consistent with these in vitro findings in cultured astrocytes, in fimbria-fornix lesioned rat hippocampus, labelling with AP-2 antibody, which specifically reacts with KPI-containing APP proteins, was mainly observed in glial fibrillary acidic protein-positive reactive astrocytes in vivo. The results showed that APP isoforms are expressed in a cell type-specific manner in the brain and, since deposition of A beta is closely associated with the expression of KPI-containing APP isoforms, provide further evidence for the involvement of astrocytes in plaque biogenesis.

Human A beta-amyloid and amyloid precursor protein accumulates in rat brain cells after cultured human leptomeningeal fibroblast implants

Cultured human leptomeningeal fibroblasts grafted into rat frontal cortex were localized to the implant pocket and to adjacent host leptomeninges. Immunohistochemical studies using a panel of human-specific and domain-specific APP antibodies revealed that all grafted cells expressed both APP and A beta in situ. Remarkably, these antibodies also labeled rat pial and ependymal cells as well as reactive astrocytes adjacent to vessels. In addition, apical projections and cell bodies of many cortical pyramidal neurons contained human-specific APP immunoreactive material. Groups of subcortical neurons, particularly those of the amygdala, hippocampal formation and suprachiasmatic nuclei, were similarly labeled. The presence of human APP in host brains was confirmed by immunoblotting. Birefringent Congo Red staining was observed in the cortical neuropil and in leptomeningeal vessels. These data indicate that grafted leptomeningeal fibroblasts hyperexpress APP and A beta which can diffuse into parenchyma and be taken up by specific rat cells.

Expression of Kunitz protease inhibitor--containing forms of amyloid beta-protein precursor within vascular thrombi

BACKGROUND

The presence of patent neovessels within vascular occlusions in chronic thromboembolic pulmonary hypertension suggests that local mechanisms exist to regulate the coagulation system. This study investigated the expression of a potent inhibitor of Factor IXa and Factor XIa (ie, protease nexin-2/ amyloid beta-protein precursor, A beta PP) in the organized vascular occlusions harvested from patients with this disease.

METHODS AND RESULTS

Immunohistochemical analysis revealed intense immunoreactivity for A beta PP in the single layer of cells that line the neovessels. A positive signal was also detected by in situ hybridization analysis with the use of a 35S-UTP-labeled antisense riboprobe that recognizes the various alternatively spliced mRNA forms of this molecule. To identify the forms of A beta PP produced within the thrombi, total RNA was extracted from the thrombi, reverse transcribed, and subjected to amplification with the use of the polymerase chain reaction (PCR) and primers that flank the region encoding the alternatively spliced 56-amino acid Kunitz-type protease inhibitor (KPI) domain. The major PCR products consisted of 255 bp and 312 bp and corresponded to transcripts encoding this domain (ie, A beta PP751 and A beta PP770). In situ hybridization analysis with the use of a 35S-UTP-labeled antisense riboprobe complementary to the region encoding the KPI domain confirmed the presence of these mRNA species in nucleated cells lining the neovessels.

CONCLUSIONS

The expression of KPI-containing isoforms of A beta PP in thrombus endothelial cells may represent one mechanism utilized in this disease to shift the local hemostatic balance and preserve regional vessel patency.

Alzheimer's disease amyloid precursor protein on the surface of cortical neurons in primary culture co-localizes with adhesion patch components

Immunofluorescence on primary dissociated rat neuronal cultures (cortical, hippocampal, and cerebellar) and organotypic hippocampal cultures was used to investigate the pattern of distribution of cell-surface amyloid protein (APP). Antibodies directed against the extracellular (N-terminal) portion of APP or against the entire molecule, but not against the C-terminal portion, revealed a striking segmental pattern of immunoreactivity along both axons and dendrites of all neuronal types tested. The pattern first developed between 24 and 48 h in culture. The segments showed co-localization with beta 1-integrin and talin immunoreactivities, but not with GAP-43 or clathrin, indicating that they may mark adhesion patches. Confocal laser microscopy supported a surface location for the APP responsible for the segmented pattern on neurites, as did the reduction of segmental immunoreactivity after exposure to mu-calpain or trypsin. It is conjectured that APP may have a role in cell-substratum interactions in the medium term, during such events as synaptic plasticity and neurite stability during extension.

Serum deprivation alters the expression and the splicing at exons 7, 8 and 15 of the beta-amyloid precursor protein in the C6 glioma cell line

Amyloid deposition characterizes the pathological lesions of Alzheimer's disease. We investigated the effect of serum deprivation on the regulation of beta-amyloid precursor protein (APP) mRNA expression in C6 glioma cells. Serum deprivation increased APP mRNA levels approximately 4-fold over controls. This increase was accompanied by changes in the pattern of alternative splicing, including the novel alternatively spliced site at exon 15. The proportion of isoforms containing exons 7 and 8 significantly increased from 61% to 68%, while isoforms lacking these exons decreased from 14% to 8%. The proportion of leukocyte-derived APP, which is a novel alternatively spliced isoform lacking exon 15, significantly increased from 19% to 40%. Among the six major isoforms produced by the two independent splicing sites, L-APP752 which contains exons 7 and 8, but lacks exon 15, increased the most (approximately 10-fold). Our findings provide evidence linking APP expression to alterations in alternative splicing at exon 15. These results demonstrate that in glial cells, APP mRNA regulation involves the alteration in alternative splicing at exons 7, 8 and 15, suggesting that not only increased expression but also an imbalance in the relative abundance of the six APP isoforms in stressed condition might affect the amyloidogenesis in Alzheimer's disease.

Two types of amyloid precursor protein (APP) mRNA in rat glioma cell lines: upregulation via a cyclic AMP-dependent pathway

APP is a multifunctional transmembrane glycoprotein and the only known natural source of beta A4 peptide-the major constituent of senile plaques in Alzheimer's disease (AD). The expression and cAMP-dependent regulation of the APP gene were investigated in primary cultures of rat astrocytes and two related glioma cell lines, BT4C and BT4Cn, which exhibit distinct invasive phenotypes. Besides the well-characterized 3.5 kb APP mRNA class, a robust expression of an unusual 2.8 kb APP mRNA class was revealed by Northern blotting in both glioma cell lines, but not in the astrocytes. Low amounts of the 2.8 kb APP mRNA species were also observed in rat liver and occasionally in aged rat brain. The 2.8 kb APP mRNA contained exons 1-18 and may thus be generated by truncation of the 3' untranslated region. For the first time, regulation of the APP gene via a cAMP-dependent mechanism was shown. Exposure to dBcAMP dramatically upregulated the 3.5 and 2.8 kb transcripts in BT4C cells, and, to a lesser extent, in BT4Cn cells where the constitutive expression of the APP gene was much higher. Elucidation of the factors involved in cAMP-dependent induction of APP mRNA in these cells may shed more light on the molecular mechanisms of APP overexpression.

Ependymins: meningeal-derived extracellular matrix proteins at the blood-brain barrier

Ependymins represent regeneration-responsive piscine glycoproteins and in many teleost fish they appear as the predominant cerebrospinal fluid constituents. Thus far, no homologous sequences have been characterized unambiguously in mammals. Sialic acid residues of the N-linked carbohydrate moiety of ependymins are responsible for their calcium-binding capacity. Ependymins from some species bear the L2/HNK-1 epitope typical of many cell adhesion molecules. After their synthesis in fibroblast-like cells of the inner endomeningeal layer, soluble ependymins are widely distributed via the cerebrospinal fluid system. Furthermore, ependymins presumably cross the intermediate endomeningeal barrier layer by way of a transcellular transport phenomenon (transcytosis). A bound form of ependymins is associated with collagen fibrils of the extracellular matrix typically found around cerebral blood vessels. Here, they might modulate the endothelial barrier function. Generally, ependymins are thought to represent a new class of possibly antiadhesive extracellular matrix proteins playing a role in specific cell contact phenomena (e.g., during regeneration).

Beta-amyloid precursor protein (APP) and APP-RNA are rapidly affected by glutamate in cultured neurons: selective increase of mRNAs encoding a Kunitz protease inhibitor domain

Alternative splicing of beta-amyloid precursor protein (APP) RNA generates APP isoforms with or without a Kunitz protease inhibitor (KPI) domain. Previously, we showed that KPI (+) APP RNA, but not KPI (-) APP RNA, is upregulated in response to experimental lesions in which neurotoxicity is dependent on NMDA receptor activation and in Alzheimer's disease hippocampus. Recent studies by Mucke et al. (1995) showed that neuronal expression of human KPI (+) APP, but not KPI (-) APP, in transgenic mice is neuroprotective against experimental lesions. In this study we examined the direct effects of the excitotoxic amino acid Glu on alternatively, spliced APP RNAs and the corresponding protein isoforms in cultured rat cortical neurons. Glu treatment rapidly induced (4.5 h) KPI (+) APP RNA but not KPI (-) APP RNA. Induction of KPI (+) RNA preceded Glu-induced neuronal cell death and was partially blocked by an NMDA-receptor antagonist. In contrast to the RNA, cellular levels of KPI (+) APP were not changed by 4.5 h of Glu treatment. Instead, the cellular full-length form of the protein KPI (-) APP was reduced by approximately 50% after 2 h of Glu treatment and remained depleted after 24 h of treatment. Cellular levels of KPI (+) forms of amyloid precursor-like protein 2 (APLP2) were not changed by Glu treatment. Our data are consistent with the hypothesis that sustained NMDA-receptor activation can regulate alternative splicing of the APP pre-mRNA in neurons.

Amyloid precursor protein (APP) expression in multiple sclerosis lesions

The amyloid precursor protein (APP) is rapidly induced in reactive glial cells in response to several pathological stimuli including inflammation. In the present study, observations previously made in animal models of autoimmune central nervous system inflammation have been extended to the analysis of multiple sclerosis (MS) lesions. A total of thirty fresh-frozen tissue blocks from six histopathologically normal control and six MS cases have been examined immunocytochemically with monoclonal antibodies directed against either C- or N-terminal epitopes of APP. Histopathological evaluation of disease progression was based on hematoxylin-eosin and oil red O staining and immunocytochemistry for T cells, macrophages/microglia, astrocytes, and oligodendrocytes. In control cases, APP immunoreactivity was generally low and confined to blood vessel walls, oligodendrocytes in white, and neurons in grey matter. In actively demyelinating plaques, however, levels of APP immunoreactivity were high, localised on T lymphocytes, foamy macrophages, activated microglia, and reactive astrocytes including astrocytic processes. In more chronic lesions, levels of APP immunoreactivity were generally lower than in acute lesions, mainly found on reactive astrocytes, their processes and a few macrophages/microglia depending on the stage of plaque development. In addition, a few 14E-positive oligodendrocytes and, moreover, numerous axons exhibited APP immunoreactivity, which was particularly pronounced with anti-C-terminal antibodies. These results demonstrate that APP is induced on reactive glial cells but also on T lymphocytes during demyelination. The extent of APP expression appears to be correlated to histopathological lesion development and thus suggests that APP detection serves as a sensitive marker for disease progression in MS.

Reactive microglia in cerebral ischaemia: an early mediator of tissue damage?

Microglial cell activation is a rapidly occurring cellular response to cerebral ischaemia. Microglia proliferate, are recruited to the site of lesion, upregulate the expression of several surface molecules including major histocompatibility complex class I and II antigens, complement receptor and the amyloid precursor protein (APP) as well as newly expressed cytokines, e.g. interleukin-1 and transforming growth factor beta 1. The ischaemia-induced production of APP may contribute to amyloid deposition in the aged brain under conditions of hypofusion. Ultrastructurally, microglia transform into phagocytes removing necrotic neurons but still respecting the integrity of eventually surviving neurons even in the close vicinity of necrotic neurons. Microglial activation starts within a few minutes after ischaemia and thus precedes the morphologically detectable neuronal damage. It additionally involves a transient generalized response within the first 24 hours post-ischaemia even at sites without eventual neuronal cell death. In functional terms, the microglial reaction appears to be a double-edged sword in ischaemia. Activated microglia may exert a cytotoxic effector function by releasing reactive oxygen species, nitric oxide, proteinases or inflammatory cytokines. All of these cytotoxic compounds may cause bystander damage following ischaemia. Pharmacological suppression of microglial activation after ischaemia has accordingly attenuated the extent of cell death and tissue damage. However, activated microglia support tissue repair by secreting factors such as transforming growth factor beta 1 which may limit tissue damage as well as suppress astroglial scar formation. In line with ultrastructural observations microglial activation in ischaemia is a strictly controlled event. By secreting cytokines and growth factors activated microglia most likely serve seemingly opposed functions in ischaemia, i.e. maintenance as well as removal of injured neurons. Post-ischaemic pharmacological modulation of microglial intervention in the cascade of events that lead to neuronal necrosis may help to improve the structural and functional outcome following CNS ischaemia.

Glial expression of the beta-amyloid precursor protein (APP) in global ischemia

The beta-amyloid precursor protein (APP) bears characteristics of an acute-phase protein and therefore is likely to be involved in the glial response to brain injury. In the brain, APP is rapidly synthesized by activated glial cells in response to comparatively mild neuronal lesions, e.g., a remote peripheral nerve injury. Perfusion deficits in the brain result largely in neuronal necrosis and are a common condition in elderly patients. This neuronal necrosis is accompanied by a pronounced reaction of astrocytes and microglia, which can also be observed in animal models. We have therefore studied in the rat, immunocytochemically, the induction of APP after 30 min of global ischemia caused by four-vessel occlusion. The postischemic brain injuries were examined at survival times from 12 h to 7 days. From day 3 onward, APP immunoreactivity was strongly induced in the CA1 and CA4 regions of the rat dorsal hippocampus as well as in the dorsolateral striatum. In these areas, the majority of APP-immunoreactive cells were reactive glial fibrillary acidic protein (GFAP)-positive astrocytes, as shown by double-immunofluorescence labeling for GFAP and APP. Additionally, small ramified cells, most likely activated microglia, expressed APP immunoreactivity. In contrast, in the parietal cortex, APP immunoreactivity occurred focally in clusters of activated microglia rather than in astrocytes, as demonstrated by double-immunofluorescence labeling for APP and the microglia-binding lectin Griffonia simplicifolia isolectin B4. In conclusion, following global ischemia, APP is induced in reactive glial cells with spatial differences in the distribution pattern of APP induction in astrocytes and microglia.

Ependymins and their potential role in neuroplasticity and regeneration: calcium-binding meningeal glycoproteins of the cerebrospinal fluid and extracellular matrix

1. Ependymins are unique, highly divergent secretory proteins of the fish endomeninx. Thus far, no homologous sequences have been characterized in mammals. 2. Soluble ependymins are the predominant constituents of the cerebrospinal fluid of many teleost fish. A bound form of these glycoproteins is associated with the extracellular matrix probably with collagen fibrils. The latter may be the functional form of ependymins. 3. Ependymins bind Ca2+ via N-linked sialic acid residues leading to a conformational transition. 4. The molecular function of ependymins seems to be related to cell contact phenomena involving the extracellular matrix. For example, adhesive or anti-adhesive interactions may possibly influence ingrowing axons.

Early and rapid de novo synthesis of Alzheimer beta A4-amyloid precursor protein (APP) in activated microglia

Upon acute activation, microglia, the immuneffector cells of the brain parenchyma, express the amyloid precursor protein (APP) that is otherwise prominent in pathological structures related to Alzheimer's disease. In this disease complex amyloid-bearing neuritic plaques contain beta A4-amyloid protein, the APP, and numerous inflammatory proteins. The accompanying activation of microglia has mostly been viewed as a secondary reaction to amyloid deposits. Activation of microglia was performed in a graded fashion. Transection of peripheral nerves such as the facial or sciatic nerve causes a microglial reaction within hours in the nucleus of origin or in projection areas of the CNS. A predominantly glial up-regulation of APP mRNA and protein could be detected as early as 6 h post lesion not only at the site of affected neuronal cell bodies but also in corresponding projection areas. Its time course suggests rapid transneuronal signalling to glial cells in the projection area. Light and electron microscopy demonstrate that microglia, which are cells of mononuclear phagocyte lineage and comprise up to 20% of all glial cells, are the dominant source for non-neuronal APP expression. Ultrastructurally, brain perivascular cells within the basal lamina constitutively express APP and thus are a possible source of vascular amyloid. Additionally, microglia express leukocyte-derived (L)-APP mRNA and protein that have recently been described in mononuclear cells of the immune system. Increased L-APP expression may serve as a potential marker for glial/microglial activation. Such immune-mediated amyloidogenesis initiated by microglia might have implications for the treatment of neurodegenerative diseases.

Expression of L-APP mRNA in brain cells

Several reports addressed the issue of how the alternative splicing of exon 7 and 8 in the APP pre-mRNA is regulated in different tissues. Of special interest here was the potential involvement of exon 7 containing APP splice isoforms, since this exon codes for a serine protease inhibitor and is therefore of putative relevance for amyloidogenic catabolism of the precursor protein. The recent identification of a third alternative splice site in close proximity to the beta A4-amyloid portion in the APP gene which may also increase APP amyloidogenicity, allowed us to investigate its regulation in cells of the central nervous system. With our assay, we were able to resolve six different APP isoforms of the eight potential isoforms which can be generated from the three alternatively spliced exons 7, 8, and 15. We demonstrate here that, in addition to rat brain microglia cells, astrocyte-enriched cultures also skip the novel alternative 3'-splice site in front of exon 15, generating L-APP mRNA. Neurons are the only cells in the central nervous system which seem to use the 3'-splice site of intron 14 nearly 100%. Interestingly, this very 3'-splice site is the only one present in the APP gene that completely matches the consensus sequence for the branchpoint sequence proposed for introns. We would therefore suggest that neurons lack a specific splicing factor which inhibits the use of the rather strong 3'-splice site in front of exon 15. It remains to be shown whether this is also the case for neurons in Alzheimer's disease.

Altered levels and splicing of the amyloid precursor protein in the adult rat hippocampus after treatment with DMSO or retinoic acid

Alzheimer's disease and cognitive impairment in rats has been associated with an increase in the percentage of amyloid precursor protein (APP) containing the KPI domain. It has recently been reported that retinoic acid (RA) is capable of increasing the levels and altering the splicing ratio of APP in cultured SH-SY5Y cells. The effects of peripherally administered RA (64 or 640 micrograms/kg; i.p.; q.d.) on the abundance of APP, the ratio of the three major isoforms, and the relative abundance of nerve growth factor (NGF), brain derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) were determined by rtPCR in the hippocampus of aged rats. Corresponding changes in choline acetyltransferase (ChAT) activity were also measured. Vehicle (DMSO) treated rats exhibited a 2 x (P < 0.01) increase in total APP and an 8 x (P < 0.001) decrease in the cyclophilin transcript. In addition, DMSO increased the percentage of APP 695 from 89% in saline treated rats to 94%. Treatment of RA in DMSO decreased the accumulation of total APP relative to cyclophilin at both the low (6.4 x; P < 0.01) and high (8 x; P < 0.05) dosages when compared to DMSO treated rats. Furthermore, the level of APP-695 decreased to 82% with low dosage of RA and 75% at high dosage of the total APP transcripts. No significant change in either NGF, NT-3, or BDNF transcripts were observed following low or high dosage RA administration relative to cyclophilin RNA nor was a change in ChAT activity detected at either of the dosages tested.(ABSTRACT TRUNCATED AT 250 WORDS)