Extracellular matrix influences the biogenesis of amyloid precursor protein in microglial cells

The Hidden Role of Non-Canonical Amyloid β Isoforms in Alzheimer's Disease

Recent advances have placed the pro-inflammatory activity of amyloid β (Aβ) on microglia cells as the focus of research on Alzheimer's Disease (AD). Researchers are confronted with an astonishing spectrum of over 100 different Aβ variants with variable length and chemical modifications. With the exception of Aβ1-42 and Aβ1-40, the biological significance of most peptides for AD is as yet insufficiently understood. We therefore aim to provide a comprehensive overview of the contributions of these neglected Aβ variants to microglia activation. First, the impact of Aβ receptors, signaling cascades, scavenger mechanisms, and genetic variations on the physiological responses towards various Aβ species is described. Furthermore, we discuss the importance of different types of amyloid precursor protein processing for the generation of these Aβ variants in microglia, astrocytes, oligodendrocytes, and neurons, and highlight how alterations in secondary structures and oligomerization affect Aβ neurotoxicity. In sum, the data indicate that gene polymorphisms in Aβ-driven signaling pathways in combination with the production and activity of different Aβ variants might be crucial factors for the initiation and progression of different forms of AD. A deeper assessment of their interplay with glial cells may pave the way towards novel therapeutic strategies for individualized medicine.

Co-Expression Analysis of microRNAs and Proteins in Brain of Alzheimer's Disease Patients

Alzheimer's disease (AD) is the most common form of dementia globally; however, the aetiology of AD remains elusive hindering the development of effective therapeutics. MicroRNAs (miRNAs) are regulators of gene expression and have been of growing interest in recent studies in many pathologies including AD not only for their use as biomarkers but also for their implications in the therapeutic field. In this study, miRNA and protein profiles were obtained from brain tissues of different stage (Braak III-IV and Braak V-VI) of AD patients and compared to matched controls. The aim of the study was to identify in the late stage of AD, the key dysregulated pathways that may contribute to pathogenesis and then to evaluate whether any of these pathways could be detected in the early phase of AD, opening new opportunity for early treatment that could stop or delay the pathology. Six common pathways were found regulated by miRNAs and proteins in the late stage of AD, with one of them (Rap1 signalling) activated since the early phase. MiRNAs and proteins were also compared to explore an inverse trend of expression which could lead to the identification of new therapeutic targets. These results suggest that specific miRNA changes could represent molecular fingerprint of neurodegenerative processes and potential therapeutic targets for early intervention.

Is kallikrein-8 a blood biomarker for detecting amnestic mild cognitive impairment? Results of the population-based Heinz Nixdorf Recall study

BACKGROUND

Kallikrein-8 (KLK8) might be an early blood-biomarker of Alzheimer's disease (AD). We examined whether blood KLK8 is elevated in persons with amnestic mild cognitive impairment (aMCI) which is a precursor of AD, compared to cognitively unimpaired (CU) controls.

METHODS

Forty cases and 80 controls, matched by sex and age (± 3years), were participants of the longitudinal population-based Heinz Nixdorf Recall study (baseline: 2000-2003). Standardized cognitive performance was assessed 5 (T1) and 10 years after baseline (T2). Cases were CU at T1 and had incidental aMCI at T2. Controls were CU at T1 and T2. Blood KLK8 was measured at T2. Using multiple logistic regression the association between KLK8 in cases vs. controls was investigated by estimating odds ratios (OR) and 95% confidence intervals (95%CI), adjusted for inter-assay variability and freezing duration. Using receiver operating characteristic (ROC) analysis, the diagnostic accuracy of KLK8 was determined by estimating the area under the curve (AUC) and 95%CI (adjusted for inter-assay variability, freezing duration, age, sex).

RESULTS

Thirty-seven participants with aMCI vs. 72 CU (36.7%women, 71.0±8.0 (mean±SD) years) had valid KLK8 measurements. Mean KLK8 was higher in cases than in controls (911.6±619.8 pg/ml vs.783.1±633.0 pg/ml). Fully adjusted, a KLK8 increase of 500pg/ml was associated with a 2.68 (1.05-6.84) higher chance of having aMCI compared to being CU. With an AUC of 0.92 (0.86-0.97), blood KLK8 was a strong discriminator for aMCI and CU.

CONCLUSION

This is the first population-based study to demonstrate the potential clinical utility of blood KLK8 as a biomarker for incipient AD.

Genetic knockdown of Klk8 has sex-specific multi-targeted therapeutic effects on Alzheimer's pathology in mice

AIMS

Previous work in our lab has identified the protease kallikrein-8 (KLK8) as a potential upstream mover in the pathogenesis of Alzheimer's disease (AD). We showed pathologically elevated levels of KLK8 in the cerebrospinal fluid and blood of patients with mild cognitive impairment or dementia due to AD, and in brains of patients and transgenic CRND8 (TgCRND8) mice in incipient stages of the disease. Furthermore, short-term antibody-mediated KLK8 inhibition in moderate stage disease alleviated AD pathology in female mice. However, it remains to be shown whether long-term reversal of KLK8 overexpression can also counteract AD. Therefore, the effects of genetic Klk8-knockdown were determined in TgCRND8 mice.

METHODS

The effects of heterozygous ablation of murine Klk8 (mKlk8) gene on AD pathology of both sexes were examined by crossbreeding TgCRND8 [hAPP+/-] with mKlk8-knockdown [mKlk8+/-] mice resulting in animals with or without AD pathology which revealed pathologically elevated or normal KLK8 levels.

RESULTS

mKlk8-knockdown had negligible effects on wildtype animals but led to significant decline of amyloid beta (Aβ) and tau pathology as well as an improvement of structural neuroplasticity in a sex-specific manner in transgenics. These changes were mediated by a shift to non-amyloidogenic cleavage of the human amyloid precursor protein (APP), recovery of the neurovascular unit and maintaining microglial metabolic fitness. Mechanistically, Klk8-knockdown improved Aβ phagocytosis in primary glia and Aβ resistance in primary neurons. Most importantly, transgenic mice revealed less anxiety and a better memory performance.

CONCLUSIONS

These results reinforce the potential of KLK8 as a therapeutic target in AD.

Extracellular Matrix Proteins Involved in Alzheimer's Disease

Alzheimer's disease (AD) is one of the most common neurodegenerative diseases and characterized by cognitive and memory impairments. Emerging evidence suggests that the extracellular matrix (ECM) in the brain plays an important role in the etiology of AD. It has been detected that the levels of ECM proteins have changed in the brains of AD patients and animal models. Some ECM components, for example, elastin and heparan sulfate proteoglycans, are considered to promote the upregulation of extracellular amyloid-beta (Aβ) proteins. In addition, collagen VI and laminin are shown to have interactions with Aβ peptides, which might lead to the clearance of those peptides. Thus, ECM proteins are involved in both amyloidosis and neuroprotection in the AD process. However, the molecular mechanism of neuronal ECM proteins on the pathophysiology of AD remains elusive. More investigation of ECM proteins with AD pathogenesis is needed, and this may lead to novel therapeutic strategies and biomarkers for AD.

CSF and blood Kallikrein-8: a promising early biomarker for Alzheimer's disease

OBJECTIVE

There is still an urgent need for supportive minimally invasive and cost-effective biomarkers for early diagnosis of Alzheimer's disease (AD). Previous work in our lab has identified Kallikrein-8 (KLK8) as a potential candidate since it shows an excessive increase in human brain in preclinical disease stages. The aim of this study was to evaluate the diagnostic performance of cerebrospinal fluid (CSF) and blood KLK8 for AD and mild cognitive impairment (MCI) due to AD.

METHODS

In this multi-centre trans-sectional study, clinical and laboratory data as well as CSF and/or blood serum samples of 237 participants, including 98 patients with mild AD, 21 with MCI due to AD and 118 controls were collected. CSF and/or serum KLK8 levels were analysed by ELISA. The diagnostic accuracy of KLK8 in CSF and blood was determined using receiver operating characteristic (ROC) analyses and compared with that of CSF core biomarkers Aβ42, P-tau and T-tau.

RESULTS

The diagnostic accuracy of CSF KLK8 was as good as that of core CSF biomarkers for AD (area under the curve (AUC)=0.89) and in case of MCI (AUC=0.97) even superior to CSF Aβ42. Blood KLK8 was a similarly strong discriminator for MCI (AUC=0.94) but slightly weaker for AD (AUC=0.83).

CONCLUSIONS

This is the first study to demonstrate the potential clinical utility of blood and CSF KLK8 as a biomarker for incipient AD. Future prospective validation studies are warranted.

Inflammatory mechanisms in neurodegeneration

This review discusses the profound connection between microglia, neuroinflammation, and Alzheimer's disease (AD). Theories have been postulated, tested, and modified over several decades. The findings have further bolstered the belief that microglia-mediated inflammation is both a product and contributor to AD pathology and progression. Distinct microglia phenotypes and their function, microglial recognition and response to protein aggregates in AD, and the overall role of microglia in AD are areas that have received considerable research attention and yielded significant results. The following article provides a historical perspective of microglia, a detailed discussion of multiple microglia phenotypes including dark microglia, and a review of a number of areas where microglia intersect with AD and other pathological neurological processes. The overall breadth of important discoveries achieved in these areas significantly strengthens the hypothesis that neuroinflammation plays a key role in AD. Future determination of the exact mechanisms by which microglia respond to, and attempt to mitigate, protein aggregation in AD may lead to new therapeutic strategies.

Higher levels of kallikrein-8 in female brain may increase the risk for Alzheimer's disease

Women seem to have a higher vulnerability to Alzheimer's disease (AD), but the underlying mechanisms of this sex dichotomy are not well understood. Here, we first determined the influence of sex on various aspects of Alzheimer's pathology in transgenic CRND8 mice. We demonstrate that beta-amyloid (Aβ) plaque burden starts to be more severe around P180 (moderate disease stage) in female transgenics when compared to males and that aging aggravates this sex-specific difference. Furthermore, we show that female transgenics suffer from higher levels of neurovascular dysfunction around P180, resulting in impaired Aβ peptide clearance across the blood-brain-barrier at P360. Female transgenics show also higher levels of diffuse microgliosis and inflammation, but the density of microglial cells surrounding Aβ plaques is less in females. In line with this finding, testosterone compared to estradiol was able to improve microglial viability and Aβ clearance in vitro. The spatial memory of transgenics was in general poorer than in wildtypes and at P360 worse in females irrespective of their genotype. This difference was accompanied by a slightly diminished dendritic complexity in females. While all the above-named sex-differences emerged after the onset of Aβ pathology, kallikrein-8 (KLK8) protease levels were, as an exception, higher in female than in male brains very early when virtually no plaques were detectable. In a second step, we quantified cerebral KLK8 levels in AD patients and healthy controls, and could ascertain, similar to mice, higher KLK8 levels not only in AD-affected but also in healthy brains of women. Accordingly, we could demonstrate that estradiol but not testosterone induces KLK8 synthesis in neuronal and microglial cells. In conclusion, multiple features of AD are more pronounced in females. Here, we show for the first time that this sex-specific difference may be meditated by estrogen-induced KLK8 overproduction long before AD pathology emerges.

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.

Regulation of Integrin α6 Recycling by Calcium-independent Phospholipase A2 (iPLA2) to Promote Microglia Chemotaxis on Laminin

Microglia are the immune effector cells that are activated in response to pathological changes in the central nervous system. Microglial activation is accompanied by the alteration of integrin expression on the microglia surface. However, changes of integrin expression upon chemoattractant (ADP) stimulation still remain unknown. In this study, we investigated whether ADP induces the alteration of integrin species on the cell surface, leading to changes in chemotactic ability on different extracellular matrix proteins. Flow cytometry scans and on-cell Western assays showed that ADP stimulation induced a significant increase of α6 integrin-GFP, but not α5, on the surface of microglia cells. Microglia also showed a greater motility increase on laminin than fibronectin after ADP stimulation. Time lapse microscopy and integrin endocytosis assay revealed the essential role of calcium-independent phospholipase A₂ activity for the recycling of α6 integrin-GFP from the endosomal recycling complex to the plasma membrane. Lack of calcium-independent phospholipase A₂ activity caused a reduced rate of focal adhesion formation on laminin at the leading edge. Our results suggest that the alteration of integrin-mediated adhesion may regulate the extent of microglial infiltration into the site of damage by controlling their chemotactic ability.

Kallikrein-8 inhibition attenuates Alzheimer's disease pathology in mice

INTRODUCTION

Memory loss and increased anxiety are clinical hallmarks of Alzheimer's disease (AD). Kallikrein-8 is a protease implicated in memory acquisition and anxiety, and its mRNA is known to be up-regulated in AD-affected human hippocampus. Therefore, an involvement of Kallikrein-8 in Alzheimer's pathogenesis is conceivable but remains to be proved.

METHODS

We determined the cerebral expression of Kallikrein-8 mRNA and protein during the course of AD in patients and in transgenic mice and tested the impact of Kallikrein-8 inhibition on AD-related pathology in mice and in primary glial cells.

RESULTS

Kallikrein-8 mRNA and protein were up-regulated in both species at incipient stages of AD. Kallikrein-8 inhibition impeded amyloidogenic amyloid-precursor-protein processing, facilitated amyloid β (Aβ) clearance across the blood-brain-barrier, boosted autophagy, reduced Aβ load and tau pathology, enhanced neuroplasticity, reversed molecular signatures of anxiety, and ultimately improved memory and reduced fear.

DISCUSSION

Kallikrein-8 is a promising new therapeutic target against AD.

Age-dependent, non-cell-autonomous deposition of amyloid from synthesis of β-amyloid by cells other than excitatory neurons

Rare, familial, early-onset autosomal dominant forms of familial Alzheimer's disease (FAD) are caused by mutations in genes encoding β-amyloid (Aβ) precursor protein (APP), presenilin-1 (PS1), and presenilin-2. Each of these genes is expressed ubiquitously throughout the CNS, but a widely held view is that excitatory neurons are the primary (or sole) source of the Aβ peptides that promote synaptic dysfunction and neurodegeneration. These efforts notwithstanding, APP and the enzymes required for Aβ production are synthesized by many additional cell types, and the degree to which those cells contribute to the production of Aβ that drives deposition in the CNS has not been tested. We generated transgenic mice in which expression of an ubiquitously expressed, FAD-linked mutant PSEN1 gene was selectively inactivated within postnatal forebrain excitatory neurons, with continued synthesis in all other cells in the CNS. When combined with an additional transgene encoding an FAD-linked APP "Swedish" variant that is synthesized broadly within the CNS, cerebral Aβ deposition during aging was found to be unaffected relative to mice with continued mutant PS1 synthesis in excitatory neurons. Thus, Aβ accumulation is non-cell autonomous, with the primary age-dependent contribution to cerebral Aβ deposition arising from mutant PS1-dependent cleavage of APP within cells other than excitatory neurons.

Complexing Aβ prevents the cellular anomalies induced by the Peptide alone

Retention of intracellular secreted APP (isAPP) can be provoked by the neurotoxic peptide Aβ. The latter decreases in the cerebrospinal fluid of Alzheimer’s disease (AD) patients, as a consequence of its cerebral accumulation and deposition into senile plaques. Of similar relevance, secreted APP (sAPP) levels can be associated with AD. The studies here presented, reinforce the link between sAPP and Aβ and address putative therapeutic strategies. Laminin and gelsolin are potential candidates; both prevent Aβ fibril formation by complexing with Aβ, thus attenuating its neurotoxicity. We show that preincubation of Aβ with laminin and gelsolin has the effect of rendering it less potent to isAPP accumulation in cortical neurons. This appears to be related to a decrease in F-actin polymerization, whereas Aβ alone induces the polymerization. Further, Aβ decreases gelsolin levels, and the latter is involved in Aβ removal. Our data indicates that Aβ-laminin and Aβ-gelsolin complexes are less neurotoxic and also less potent than fibrillar Aβ at inducing isAPP retention. These results validate the potential of these proteins as therapeutic strategies that prevent the Aβ-induced effects. In hence, given that Aβ decreases the levels of proteins involved in its own clearance, this may contribute to the mechanisms underlying AD pathology.

Microglia are the major source of TNF-α and TGF-β1 in postnatal glial cultures; regulation by cytokines, lipopolysaccharide, and vitronectin

Damage to the central nervous system (CNS) leads to increased production of TNF-α and TGF-β1 cytokines that have pro- or anti-inflammatory actions, respectively. To define whether astrocytes or microglia express these cytokines, prior studies have used mixed glial cultures (MGC) to represent astrocytes, thought these results are inevitably complicated by the presence of contaminating microglia within MGC. To clarify the cellular source of these cytokines, here we employed a recently described method of preparing microglia-free astrocyte cultures, in which neural stem cells (NSC) are differentiated into astrocytes. Using ELISA to quantify cytokine production in three types of glial culture: MGC, pure microglia or pure astrocytes, this showed that microglia but not astrocytes, produce TNF-α, and that this expression is increased by LPS, IFN-γ, and to a lesser extent by vitronectin, but decreased by TGF-β1. In contrast, TGF-β1 was produced by microglia and astrocytes, though at 10-fold higher levels by microglia. TGF-β1 expression in microglia was increased by vitronectin and to a lesser extent by TNF-α and LPS, but astrocyte TGF-β1 expression was not regulated by any factor tested. In summary, our data reveal that microglia, not astrocytes are the major source of TNF-α and TGF-β1 in postnatal glial cultures, and that microglial production of these antagonistic cytokines is tightly regulated by cytokines, LPS, and vitronectin.

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.

The upside of APP at synapses

The memory dysfunctions that characterize Alzheimer's disease (AD) are strongly correlated with synapse loss. The amyloid precursor protein (APP) and its cleavage product Aβ play central roles in synapse and memory loss, and thus are strongly implicated in the pathogenesis of AD. Numerous in vitro and transgenic AD mouse model studies have shown that overexpression of APP leads to Aβ accumulation, which causes decreased synaptic activity and dendritic spine density. However, the normal synaptic function of APP itself is not fully understood. Several recent studies have found that full-length APP promotes synaptic activity, synapse formation, and dendritic spine formation. These findings cast APP as a potential key player in learning and memory. It is of interest that the synaptic functions of full-length APP are opposite to the effects associated with pathological Aβ accumulation. In this review, we will summarize the normal functions of APP at synapses and spines along with other known functions of APP, including its role in cell motility, neuronal migration, and neurite outgrowth. These studies shed light on the physiological actions of APP, independent of Aβ effects, and thus lead to a better understanding of the synaptic dysfunctions associated with AD.

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.

Adherence-dependent shifts in the patterns of beta-amyloid peptides secreted by human mononuclear phagocytes

Cells of the mononuclear phagocyte system are closely associated with vascular and neuritic beta-amyloid deposits in Alzheimer's disease. Using one-dimensional and newly developed two-dimensional Abeta-SDS-PAGE Western immunoblot techniques (1D/2D-Abeta-WIB) we investigated the patterns of Abeta peptides released by primary non-adherent and adherence-activated human mononuclear phagocytes in vitro. An overall increase of total released Abeta peptides (Abeta(total)) was observed in adherence-activated mononuclear phagocyte cultures. 2D-Abeta-WIB revealed that the proportion of Abeta(1-40) decreased significantly to 50.2+/-5.4% (n=10) of Abeta(total) compared to 65.9+/-5.6% (n=7) in non-adherent cultures (p<0.0001, t=5.82). Abeta(1-42) accounted for only 3.0+/-2.1% of Abeta(total) and its proportion did not change significantly upon adherence (2.8+/-0.5% of Abeta(total)). In adherence-activated cultures we detected pronounced shifts in the fractional pattern of released Abeta peptides in favour of N-truncated species. The second most prominent Abeta peptide accounted for as much as 12.7+/-3.0% of Abeta(total) (2.0+/-1.2% in non-adherent cultures; p<0.0001, t=9.00) and was identified as Abeta(2-40) by comigration with a synthetic peptide and by N-terminal-specific antibodies. A strong increase of a further Abeta immunoreactive spot migrating at pI 5.45 was observed. It accounted for 9.2+/-1.7% of Abeta(total) as compared to 1.0+/-0.9% in non-adherent cultures (p<0.0001, t=11.61) and presumably represented a variant of Abeta(2-40) as determined by C-terminal Abeta(40)-specific immunoprecipitation and N-terminal-specific immunodetection. Thus, mononuclear phagocytes might be one source of the N-truncated Abeta peptides regularly found in human plasma and are less likely to contribute substantially to plasma Abeta(1-42).

Differences in extracellular matrix production and basic fibroblast growth factor response in skin fibroblasts from sporadic and familial Alzheimer's disease

Extracellular matrix (ECM) molecules and growth factors, such as fibroblast growth factor (FGF), play a crucial role in Alzheimer's disease (AD). The purpose of this investigation was to determine whether phenotypic alterations in ECM production are present in non-neuronal AD cells associated with different FGF expression and response. Synthesis of glycosaminoglycans (GAG) and collagen were measured in skin fibroblasts from patients with familial, sporadic AD (FAD and SAD respectively), and from age-matched controls by radiolabeled precursors. Proteoglycans (PG), metalloprotease (MMP)-1, and FGF gene expressions were measured by reverse transcription-polymerase chain reaction. The results showed different ECM neosynthesis and mRNA levels in the two AD fibroblast populations. FAD accumulated more collagen and secreted less GAG than SAD. Biglycan PG was upregulated in FAD while betaglycan, syndecan, and decorin were markedly downregulated in SAD fibroblasts. We found a significant decrease of MMP1, more marked in FAD than in SAD fibroblasts. Constitutive FGF expression was greatly reduced in both pathological conditions (SAD>FAD). Moreover, an inverse high affinity/low affinity FGF receptor ratio between SAD and FAD fibroblasts was observed. FGF treatment differently modulated ECM molecule production and gene expression in the two cell populations. These observations in association with the changes in FGF gene expression and in the FGF receptor number, suggest that cellular mechanisms downstream from FGF receptor binding are involved in the two different forms of AD.

An artificial three-dimensional matrix promotes ramification in the microglial cell-line, BV-2

Three-dimensional (3D) cell culture yields strikingly different cell phenotypes compared to two-dimensional (2D) cell culture. Since microglia, monocyte derived immune cells in the brain, exist in a variety of cell shapes ranging from amoeboid to ramified, we evaluated the impact of 2D versus 3D culture conditions on cell shape. The microglial cell-line, BV-2, was either cultured on poly-D-lysine coated dishes (2D culture conditions) or in a BD Pura Matrix Peptide Hydrogel (3D culture conditions) in the absence or presence of the extracellular matrix proteins, fibronectin and collagen type I, respectively. We identified five distinct morphological phenotypes (amoeboid, bipolar, tripolar, multipolar, ramified) and compared the frequency distribution of these phenotypes under different culture conditions using a chi(2) test. Culture of BV-2 cells in an inert 3D matrix shifted the frequency distribution from an amoeboid dominated population, which is typical for BV-2 cells cultured under conventional 2D conditions, to a population dominated by multipolar phenotypes. Fibronectin or collagen type I significantly suppressed matrix-induced ramification. These cell culture experiments illustrate the dependency of cell shape on spatial distribution of potential adhesion sites.

Reactive microgliosis participates in MPP+-induced dopaminergic neurodegeneration: role of 67 kDa laminin receptor

It has been reported that extracellular matrix (ECM) molecules regulate monocyte activation by binding with a 67 kDa nonintegrin laminin receptor (LR). As microgliosis is a pivotal factor in propelling the progress of chronic neurodegeneration in the brain, we hypothesized that LR may regulate the microgliosis and subsequent neurotoxicity. Using 1-methyl-4-phenylpyridinium (MPP+) -treated C57 mice primary mesencephalic neuron-glia cultures as an in vitro Parkinson's disease (PD) model, we observed that MPP+ treatment increased LR expression only in the mixed neuron-glia but not in microglia-enriched or microglia-depleted cultures, indicating that MPP+-induced increase of LR expression is associated with neuron-microglia interaction. Using confocal microscopic examination, we found that LR was localized in the microglia, which were F4/80 positive. Treatment with the antibody (Ab) against LR (LR-Ab) or YIGSR, a synthetic pentapeptide inhibitor for LR, significantly attenuated the MPP+-increased F4/80 immunoreactivity (24 h) and dopaminergic (DA) neurotoxicity. LR-Ab also attenuated MPP+-increased microglial phagocytotic activity (48 h) and the superoxide production (4 days). Further study demonstrated that exogenous laminin (1-10 microg/ml) treatment induced microglial activation and DA neurotoxicity, in a dose-dependent manner, which was partially attenuated by the LR-Ab. We concluded that by regulating cell-ECM interaction, LR plays important roles in mediating microgliosis and subsequent DA neurotoxicity. Laminin is a potential ligand for activating this LR receptor. This study also suggests that laminin/LR is a potential target for developing new therapeutic drugs against neurodegenerative disorders such as PD.

Amyloid precursor protein cross-linking stimulates beta amyloid production and pro-inflammatory cytokine release in monocytic lineage cells

Beta amyloid peptide-containing neuritic plaques are a defining feature of Alzheimer's disease pathology. Beta amyloid are 38-43 residue peptides derived by proteolytic cleavage of amyloid precursor protein. Although much attention has focused on the proteolytic events leading to beta amyloid generation, the function of amyloid precursor protein remains poorly described. Previously, we reported that amyloid precursor protein functions as a pro-inflammatory receptor on monocytic lineage cells and defined a role for amyloid precursor protein in adhesion by demonstrating that beta(1) integrin-mediated pro-inflammatory activation of monocytes is amyloid precursor protein dependent. We demonstrated that antibody-induced cross-linking of amyloid precursor protein in human THP-1 monocytes and primary mouse microglia stimulates a tyrosine kinase-based pro-inflammatory signaling response leading to acquisition of a reactive phenotype. Here, we have identified pro-inflammatory mediators released upon amyloid precursor protein-dependent activation of monocytes and microglia. We show that amyloid precursor protein cross-linking stimulated tyrosine kinase-dependent increases in pro-inflammatory cytokine release and a tyrosine kinase-independent increase in beta amyloid 1-42 generation. These data provide much needed insight into the function of amyloid precursor protein and provide potential therapeutic targets to limit inflammatory changes associated with the progression of Alzheimer's disease.

Endoproteolytic Cleavage of FE65 Converts the Adaptor Protein to a Potent Suppressor of the sAPPα Pathway in Primates

Adaptor protein FE65 (APBB1) specifically binds to the intracellular tail of the type I transmembrane protein, beta-amyloid precursor protein (APP). The formation of this complex may be important for modulation of the processing and function of APP. APP is proteolytically cleaved at multiple sites. The cleavages and their regulation are of central importance in the pathogenesis of dementias of the Alzheimer type. In cell cultures and perhaps in vivo, secretion of the alpha-cleaved APP ectodomain (sAPPalpha) is the major pathway in the most cells. Regulation of the process may require extracellular/intracellular cues. Neither extracellular ligands nor intracellular mediators have been identified, however. Here, we show novel evidence that the major isoform of FE65 (97-kDa FE65, p97FE65) can be converted to a 65-kDa N-terminally truncated C-terminal fragment (p65FE65) via endoproteolysis. The cleavage region locates immediately after an acidic residue cluster but before the three major protein-protein binding domains. The cleavage activity is particularly high in human and non-human primate cells and low in rodent cells; the activity appears to be triggered/enhanced by high cell density, presumably via cell-cell/cell-substrate contact cues. As a result, p65FE65 exhibits extraordinarily high affinity for APP (up to 40-fold higher than p97FE65) and potent suppression (up to 90%) of secretion of sAPPalpha. Strong p65FE65-APP binding is required for the suppression. The results suggest that p65FE65 may be an intracellular mediator in a signaling cascade regulating alpha-secretion of APP, particularly in primates.

Amyloid precursor protein expression in circulating monocytes and brain macrophages from patients with HIV-associated cognitive impairment

We examined amyloid precursor protein (APP) surface expression on circulating leukocytes and in brain tissues from normal individuals and HIV+ subjects with cognitive impairment. Most monocytes, and a subset of B-lymphocytes, expressed APP, while T-lymphocytes, granulocytes, and natural killer (NK) cells did not. CD14bright/CD16+ monocytes expressed the highest levels, and CD14dim/CD16+ cells were negative, suggesting a relationship with activation. Higher APP+ monocyte levels correlated with increased numbers of CD16+ monocytes, but not with the degree of cognitive impairment. Treatment of monocytes with M-CSF, but not LPS, upregulated APP expression. In the brain, APP appeared as axonal immunoreactivity and diffuse plaques, and APP+ perivascular macrophages were seen in cases with severe dementia. APP may facilitate monocyte entry into the brain.

The extracellular matrix and cytokines regulate microglial integrin expression and activation

Microglia are the primary immune effector cells resident within the CNS, whose activation into migratory, phagocytic cells is associated with increased expression of cell adhesion molecules of the integrin family. To determine which specific factors are important regulators of microglial activation and integrin expression, we have examined the influence of individual cytokines and extracellular matrix (ECM) substrates by quantifying cell surface expression of MHC and individual integrins by flow cytometry. We found that the proinflammatory cytokines TNF and IFN-alpha promoted microglial activation, as assessed by amoeboid morphology and increased expression of MHC class I, and also increased expression of the alpha(4)beta(1) and Mac-1 integrins. In contrast, TGF-beta1 had the opposite effect and was dominant over the other cytokines. Furthermore, the ECM substrates fibronectin and vitronectin, but not laminin, also promoted microglial activation and increased expression of the alpha(4)beta(1), alpha(5)beta(1) and Mac-1 integrins, but significantly, the influence of fibronectin and vitronectin was not diminished by TGF-beta1. Taken together, this work suggests that, in addition to cytokines, the ECM represents an important regulatory influence on microglial activity. Specifically, it implies that increases in the local availability of fibronectin or vitronectin, as a result of blood-brain barrier breakdown or increased expression in different pathological states of the CNS, could induce microglial activation and increased expression of integrins.

Neuroplasticity in Alzheimer's disease

Ramon y Cajal proclaimed in 1928 that "once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. In the adult centers the nerve paths are something fixed, ended and immutable. Everything must die, nothing may be regenerated. It is for the science of the future to change, if possible, this harsh decree." (Ramon y Cajal, 1928). In large part, despite the extensive knowledge gained since then, the latter directive has not yet been achieved by 'modern' science. Although we know now that Ramon y Cajal's observation on CNS plasticity is largely true (for lower brain and primary cortical structures), there are mechanisms for recovery from CNS injury. These mechanisms, however, may contribute to the vulnerability to neurodegenerative disease. They may also be exploited therapeutically to help alleviate the suffering from neurodegenerative conditions.

The integrin family of cell adhesion molecules has multiple functions within the CNS

Integrins comprise a large family of cell adhesion molecules that mediate interactions between the extracellular environment and the cytoplasm. During the last decade, analysis of the expression and function of these molecules has revealed that integrins regulate many aspects of cell behavior including cell death, proliferation, migration, and differentiation. Within the central nervous system (CNS), most of the early studies focused on the role of integrins in mediating adhesive and migratory events in two distinct processes: neural development and CNS inflammation. Interestingly, recent analysis of transgenic mice has provided some surprising results regarding the role of integrins in neural development. Furthermore, a large body of evidence now supports the idea that in addition to these well-described functions, integrins play multiple roles in the CNS, both during development and in the adult in areas as diverse as synaptogenesis, activation of microglia, and stabilization of the endothelium and blood-brain barrier. Many excellent reviews have addressed the contribution of integrins in mediating leukocyte extravasation during CNS inflammation. This review will focus on recently emerging evidence of novel and diverse roles of integrins and their ligands in the CNS during development and in the adult, in health and disease.

Transcriptional activation and increase in expression of Alzheimer's beta-amyloid precursor protein gene is mediated by TGF-beta in normal human astrocytes

The overexpression of the Alzheimer amyloid precursor protein (APP) and its subsequent proteolytic processing may be one of several factors contributing to amyloid beta-peptide (Abeta) deposition in plaques and microvasculature in Alzheimer's disease (AD) brain. Cytokines and growth factors can influence the expression of APP in response to brain injury, but the underlying mechanisms are largely unknown. We examined the mechanisms by which transforming growth factor-beta (TGF-beta) affects the expression of APP in normal human astrocytes. We report that, TGF-beta up-regulated the expression of APP at the transcription level as determined by nuclear run-on experiments. Transient transfection of astrocytes with APP gene promoter (-2832 bp) chloramphenicol acetyltransferase (CAT) reporter constructs led to increased reporter activity upon TGF-beta stimulation. This reporter activity was mainly attributed to the APP proximal domain (-488 bp). The increase in APP gene transcription was associated with significant accumulation of intracellular APP, APP carboxyl terminal derived fragments, and total secreted Abeta. In addition, we observed a significant increase in levels of TGF-beta in Abeta plaques and its immediate vicinity in AD-affected brain relative to controls. These results indicate that high levels of TGF-beta in the cortex, may serve to up-regulate APP synthesis in reactive astrocytes and indirectly contributes to Abeta deposition. Closely related processes may induce cerebrovascular pathology in AD brain.

Transforming growth factor-beta-induced transcription of the Alzheimer beta-amyloid precursor protein gene involves interaction between the CTCF-complex and Smads

Transforming growth factor-beta-1 (TGF-beta), a key regulator of the brain responses to injury and inflammation, has been implicated in upregulating the expression of the Alzheimer amyloid precursor protein (APP) and Alzheimer's disease (AD) pathogenesis. However, little is known about the mechanisms underlying the effects of TGF-beta on APP expression. Analysis of APP promoter activity upstream of the chloramphenicol acetyltransferase reporter gene in normal human astrocytes (NHAs), revealed that the APP promoter binding beta (APBbeta) site (-93/-82) is responsive to TGF-beta. This site interacts with the zinc finger nuclear factor CTCF, involved in APP transcriptional activity. As determined by gel shift assay, there was no significant difference in the CTCF-APBbeta complex binding activity in the presence or absence of TGF-beta treatment of NHAs. To further investigate the contributions of the CTCF-complex and Smad proteins to the TGF-beta induced APP promoter activity, we examined the distribution of these factors and their DNA binding activity. Interestingly, upon TGF-beta treatment both Smads 3 and 4 were translocated to the nuclei in contrast to Smad 2, which was cytoplasmic. However, CTCF was predominantly localized in the nuclei irrespective of TGF-beta treatment. Gel super shift assay coupled with Western blot analysis showed that Smads 3 and 4 specifically associated with the CTCF-APBbeta complex. In addition, AD brain sections showed increased expression and nuclear localization of Smad 4, which correlated with higher levels of APP and TGF-beta. However, over expression of Smad 4 on its own was not sufficient to affect APP expression. These results demonstrate that TGF-beta activation of Smad protein complexes promotes transcription of the APP gene. Increased synthesis of APP may in part determine Abeta production and deposition in affected AD brain.

Differential distribution of laminins in Alzheimer disease and normal human brain tissue

Immunocytochemistry, Western blotting, and RT-PCR were used to identify the isoforms of laminin expressed in the Alzheimer disease, but not in normal human brain tissue. We found that alpha 1 laminin was heavily over-expressed in Alzheimer disease frontal cortex, and localized in reactive astrocytes of the grey and white matter, and as punctate deposits in the senile placques of the Alzheimer brain tissue. Antibodies against the C-terminal neurite outgrowth domain of the gamma 1 laminin demonstrated expression of the gamma 1 laminin in GFAP-immunoreactive reactive astrocytes of the Alzheimer disease frontal cortex. The gamma 1 laminin was also heavily over-expressed in reactive astrocytes of both grey and white matter. Although antibodies against the C-terminal neurite outgrowth domain failed to localize gamma 1 laminin in senile plaques, antibodies against the N-terminal domains of the gamma 1 laminin demonstrated gamma 1 laminin as punctate deposits in the senile plaques. The present results indicate that enhanced and specialized expression patterns of alpha 1 and gamma 1 laminins distinctly associate these two laminins with the Alzheimer disease. The fact that domain specific antibodies localize both alpha1 and gamma 1 laminins in the senile plaques as punctate deposits and in astrocytes of both the gray and white matter indicate that these laminins and their specific domains may have distinct functions in the pathophysiology of the Alzheimer disease.

Fibronectin modulation by A beta amyloid peptide (25-35) in cultured astrocytes of newborn rat cortex

Fibronectin appears to be present in Senile Plaques of Alzheimer's disease brains. These senile or neuritic plaques are surrounded by dystrophic neurites, activated microglia and reactive astrocytes. The purpose of this work was to establish if a direct correlation exists between the production of Fibronectin (FN) by astrocytes and the presence of amyloid, analysing the modification of this protein produced after the treatment of cultured astrocytes with amyloid peptide (25-35). Our data showed that the addition of previously polymerised A beta-peptide to cultured astrocytes induced a marked increase in FN immunoreactivity that is in part dependent on phosphatases 2A or phosphatase 1, since was partially inhibited by okadaic acid. The increased amount of FN did not appear to be associated to any specific single isoform of which are mainly present in the rat brain. Our data suggest that in vivo FN accumulated in senile plaques may be the result, at least in part, of the response of reactive astrocyte to the presence of amyloid peptide. The importance of FN up-regulation in vivo, as part of a 'positive' response of the astrocytes to produce molecules that favours neurite outgrowth, is discussed.

Microglial chemotaxis, activation, and phagocytosis of amyloid beta-peptide as linked phenomena in Alzheimer's disease

Microglia are widely held to play important pathophysiologic roles in Alzheimer's disease (AD). On exposure to amyloid beta peptide (A beta) they exhibit chemotactic, phagocytic, phenotypic and secretory responses consistent with scavenger cell activity in a localized inflammatory setting. Because AD microglial chemotaxis, phagocytosis, and secretory activity have common, tightly linked soluble intermediaries (e.g., cytokines, chemokines), cell surface intermediaries (e.g., receptors, opsonins), and stimuli (e.g., highly inert A beta deposits and exposed neurofibrilly tangles), the mechanisms for microglial clearance of A beta are necessarily coupled to localized inflammatory mechanisms that can be cytotoxic to nearby tissue. This presents a critical dilemma for strategies to remove A beta by enhancing micoglial activation--a dilemma that warrants substantial further investigation.

Microglial signalling cascades in neurodegenerative disease

Activated microglia release a number of substances, the specific cocktail released depending on the stimulus. Many of the substances released by microglia also serve to activate them, suggesting the presence of a number of autocrine/paracrine loops. Because of the low density of microglia present in the normal brain, such autocrine/paracrine loops may not be significant but during the initiation and ongoing states of neurodegeneration, the increased concentrations of microglia may allow the activation and escalated stimulation of these feedback pathways. The activation of p38 MAPK by A beta and cytokines may be part of a microglial autocrine loop which results in the fueling of the microglial inflammatory response. A novel class of cytokine suppressive anti-inflammatory drugs (CSAIDs) inhibit the activation of p38 kinase (Bhat et al., 1998) suggesting this kinase plays a key role in transducing microglial responses to activation stimuli (Badger et al., 1996).

The functions of the amyloid precursor protein gene

The amyloid precursor protein (APP) gene and its protein products have multiple functions in the central nervous system and fulfil criteria as neuractive peptides: presence, release and identity of action. There is increased understanding of the role of secretases (proteases) in the metabolism of APP and the production of its peptide fragments. The APP gene and its products have physiological roles in synaptic action, development of the brain, and in the response to stress and injury. These functions reveal the strategic importance of APP in the workings of the brain and point to its evolutionary significance.

Interactions of laminin with the amyloid beta peptide. Implications for Alzheimer's disease

Extensive neuronal cell loss is observed in Alzheimer's disease. Laminin immunoreactivity colocalizes with senile plaques, the characteristic extracellular histopathological lesions of Alzheimer brain, which consist of the amyloid beta (A(beta)) peptide polymerized into amyloid fibrils. These lesions have neurotoxic effects and have been proposed to be a main cause of neurodegeneration. In order to understand the pathological significance of the interaction between laminin and amyloid, we investigated the effect of laminin on amyloid structure and toxicity. We found that laminin interacts with the A(beta)1-40 peptide, blocking fibril formation and even inducing depolymerization of preformed fibrils. Protofilaments known to be intermediate species of A(beta) fibril formation were also detected as intermediate species of laminin-induced A(beta) fibril depolymerization. Moreover, laminin-amyloid interactions inhibited the toxic effects on rat primary hippocampal neurons. As a whole, our results indicate a putative anti-amyloidogenic role of laminin which may be of biological and therapeutic interest for controlling amyloidosis, such as those observed in cerebral angiopathy and Alzheimer's disease.

Laminins and human disease

The laminin protein family has diverse tissue expression patterns and is involved in the pathology of a number of organs, including skin, muscle, and nerve. In the skin, laminins 5 and 6 contribute to dermal-epidermal cohesion, and mutations in the constituent chains result in the blistering phenotype observed in patients with junctional epidermolysis bullosa (JEB). Allelic heterogeneity is observed in patients with JEB: mutations that results in premature stop codons produce a more severe phenotype than do missense mutations. Gene therapy approaches are currently being studied in the treatment of this disease. A blistering phenotype is also observed in patients with acquired cicatricial pemphigoid (CP). Autoantibodies targeted against laminins 5 and 6 destabilize epithelial adhesion and are pathogenic. In muscle cells, laminin alpha 2 is a component of the bridge that links the actin cytoskeleton to the extracellular matrix. In patients with laminin alpha 2 mutations, the bridge is disrupted and mature muscle cells apoptose. Congenital muscular dystrophy (CMD) results. The role of laminin in diseases of the nervous system is less well defined, but the extracellular protein has been shown to serve an important role in peripheral nerve regeneration. The adhesive molecule influences neurite outgrowth, neural differentiation, and synapse formation. The broad spatial distribution of laminin gene products suggests that laminin may be involved in a number of diseases for which pathogenic mechanisms are still being unraveled.

Laminin 1 attenuates beta-amyloid peptide Abeta(1-40) neurotoxicity of cultured fetal rat cortical neurons

A growing amount of evidence indicates the involvement of extracellular matrix components, especially laminins, in the development of Alzheimer's disease, although their role remains unclear. In this study, we clearly demonstrate that laminin 1 inhibits beta-amyloid peptide (Abeta)-induced neuronal cell death by preventing the fibril formation and interaction of the Abeta peptide with cell membranes. The presence of laminin at a laminin/Abeta peptide molar ratio of 1:800 significantly inhibits the Abeta-induced apoptotic events, together with inhibition of amyloid fibril formation. The inhibitory effects of laminin 1 were time- and dose-dependent, whereas laminin 2 had less effect on Abeta neurotoxicity. A preincubation of laminin and Abeta was not required to observe the protective effect of laminin, suggesting a direct interaction between laminin 1 and Abeta. Moreover, laminin had no effect on the toxicity of the fibrillar Abeta peptide, suggesting an interaction of laminin with nonfibrillar species of the Abeta peptide, sequestering the peptide in a soluble form. These data extend our understanding of laminin-dependent binding of Abeta and highlight the possible modulation role of laminin regarding Abeta aggregation and neurotoxicity in vivo.

Effects of transforming growth factor-beta (isoforms 1-3) on amyloid-beta deposition, inflammation, and cell targeting in organotypic hippocampal slice cultures

The transforming growth factor-beta (TGF-beta) family consists of three isoforms and is part of a larger family of cytokines regulating differentiation, development, and tissue repair. Previous work from our laboratory has shown that TGF-beta1 can increase amyloid-beta protein (Abeta) immunoreactive (Abetair) plaque-like deposits in rat brain. The aim of the current study was to evaluate all three isoforms of TGF-beta for their ability to affect the deposition and neurotoxicity of Abeta in an organotypic, hippocampal slice culture model of Abeta deposition. Slice cultures were treated with Abeta either with or without one of the TGF-beta isoforms. All three isoforms can increase Abeta accumulation (over Abeta treatment alone) within the slice culture, as determined by ELISA. However, there are striking differences in the pattern of Abetair among the three isoforms of TGF-beta. Isoforms 1 and 3 produced a cellular pattern of Abeta staining that colocalizes with GS lectin staining (microglia). TGF-beta2 produces dramatic Abeta staining of pyramidal neurons in layers CA1-CA2. In addition to cellular Abeta staining, plaque-like deposits are increased by all of the TGF-betas. Although no gross toxicity was observed, morphological neurodegenerative changes were seen in the CA1 region when the slices were treated with Abeta plus TGF-beta2. Our results demonstrate important functional differences among the TGF-beta isoforms in their ability to alter the cellular distribution and degradation of Abeta. These changes may be relevant to the pathology of Alzheimer's disease (AD).

Microglia and Alzheimer's disease

Microglia play a major role in the cellular response associated with the pathological lesions of Alzheimer's disease. As brain-resident macrophages, microglia elaborate and operate under several guises that seem reminiscent of circulating and tissue monocytes of the leucocyte repertoire. Although microglia bear the capacity to synthesize amyloid beta, current evidence is most consistent with their phagocytic role. This largely involves the removal of cerebral amyloid and possibly the transformation of amyloid beta into fibrils. The phagocytic functions also encompass the generation of cytokines, reactive oxygen and nitrogen species, and various proteolytic enzymes, events that may exacerbate neuronal damage rather than incite outgrowth or repair mechanisms. Microglia do not appear to function as true antigen-presenting cells. However, there is circumstantial evidence that suggests functional heterogeneity within microglia. Pharmacological agents that suppress microglial activation or reduce microglial-mediated oxidative damage may prove useful strategies to slow the progression of Alzheimer's disease.

Importance of immunological and inflammatory processes in the pathogenesis and therapy of Alzheimer's disease

The contribution of autoimmune processes or inflammatory components in the etiology and pathogenesis of Alzheimer's disease (AD) has been suspected for many years. The presence of antigen-presenting, HLA-DR-positive and other immunoregulatory cells, components of complement, inflammatory cytokines and acute phase reactants have been established in tissue of AD neuropathology. Although these data do not confirm the immune response as a primary cause of AD, they indicate involvement of immune processes at least as a secondary or tertiary reaction to the preexisting pathogen and point out its driving-force role in AD pathogenesis. These processes may contribute to systemic immune response. Thus, experimental and clinical studies indicate impairments in both humoral and cellular immunity in an animal model of AD as well as in AD patients. On the other hand, anti-inflammatory drugs applied for the treatment of some chronic inflammatory diseases have been shown to reduce risk of AD in these patients. Therefore, it seems that anti-inflammatory drugs and other substances which can control the activity of immunocompetent cells and the level of endogenous immune response can be valuable in the treatment of AD patients.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.

The distribution of beta-amyloid precursor protein in rat cortex after systemic kainate-induced seizures

In the current study we employed immunohistochemical techniques to identify neuronal and glial cells in specific brain areas that modulate beta-amyloid precursor protein (betaAPP) synthesis following kainate-induced seizures. In addition, antibodies directed against the FOS protein, which is generated by activation of the immediate early gene c-fos and is temporally associated with ongoing seizure activity, were used to identify transneuronal pathways activated after kainate-induced seizures (KIS). It was therefore possible to correlate the appearance of activated neuronal pathways identified by FOS-like immunoreactivity (LI) and PAPP-LI in alternate sections. In addition, we employed immunohistochemical procedures to characterize morphological changes in neuronal and glial cells following kainate-induced seizures in both young and adult rats. Our results demonstrate a specific pattern of FOS-LI induced by kainate injection. In older animals FOS-LI spreads out from limbic cortical regions, including the piriform and entorhinal cortex, to other cortical regions, including the parietal and somatosensory cortices. Seizures were associated with decrease in neuronal betaAPP-LI in both young and adult rats, whereas glial betaAPP-LI markedly increased. The increase in betaAPP-LI glia was far more extensive in adult than in young rats and the anatomical distribution of betaAPP-LI glia was grossly correlated with FOS-LI. The spread of betaAPP-LI follows seizure-activated transsynaptic pathways. It is likely that the sequence of events following kainate injection is initially triggered by c-fos gene expression, which is rapidly followed by modulation of betaAPP synthesis in parallel to, or preceding, morphological changes of both microglia and astrocytes. The present study, which extensively characterized early changes in c-fos expression and betaAPP-LI in glia following kainate-induced seizures, is a potentially useful animal model for the in vivo study of numerous facets of betaAPP synthesis and the possible role of such processes in Alzheimer's disease.

Distinct sites of intracellular production for Alzheimer's disease A beta40/42 amyloid peptides

The Alzheimer amyloid precursor protein (APP) is cleaved by several proteases, the most studied, but still unidentified ones, are those involved in the release of a fragment of APP, the amyloidogenic beta-protein A beta. Proteolysis by gamma-secretase is the last processing step resulting in release of A beta. Cleavage occurs after residue 40 of A beta [A beta(1-40)], occasionally after residue 42 [A beta(1-42)]. Even slightly increased amounts of this A beta(1-42) might be sufficient to cause Alzheimer's disease (AD) (reviewed in ref. 1, 2). It is thus generally believed that inhibition of this enzyme could aid in prevention of AD. Unexpectedly we have identified in neurons the endoplasmic reticulum (ER) as the site for generation of A beta(1-42) and the trans-Golgi network (TGN) as the site for A beta(1-40) generation. It is interesting that intracellular generation of A beta seemed to be unique to neurons, because we found that nonneuronal cells produced significant amounts of A beta(1-40) and A beta(1-42) only at the cell surface. The specific production of the critical A beta isoform in the ER of neurons links this compartment with the generation of A beta and explains why primarily ER localized (mutant) proteins such as the presenilins could induce AD. We suggest that the earliest event taking place in AD might be the generation of A beta(1-42) in the ER.

Individual isoforms of the amyloid beta precursor protein demonstrate differential adhesive potentials to constituents of the extracellular matrix

The amyloid beta precursor protein (AbetaPP) can exist as a membrane-bound glycoprotein which modulates neural cell adhesion. The adhesion of clones of the AtT20 mouse pituitary cell line, transfected with cDNA coding for the 695 (AbetaPP695) and 751 (AbetaPP751) amino acid forms of the protein, to individual components of the extracellular matrix was determined using a centrifugal shear assay. On laminin, poly-L-lysine, fibronectin, and uncoated glass substrata, the cells transfected with AbetaPP695 (6A1 cells) demonstrated a 50% increase in adhesivity over nontransfected cells, while those transfected with AbetaPP751 (7A1 cells) showed a significant decrease in adhesion. There was, however, a significant increase in the adhesive strength of the 7A1 cells to collagen type IV with no change in the adhesivity of the 6A1 cells when compared with control. These changes in adhesivity could be attributed to changes in the levels of the membrane-bound protein and were not due to the interaction of soluble AbetaPP with elements of the extracellular matrix. These studies provide evidence for differential adhesivities of the constituent AbetaPP isoforms and the possible role of the Kunitz protease inhibitor (KPI) domain in influencing the adhesive properties of the protein backbone.

Astrogliosis in the neonatal and adult murine brain post-trauma: elevation of inflammatory cytokines and the lack of requirement for endogenous interferon-gamma

The relevance of astrogliosis remains controversial, especially with respect to the beneficial or detrimental influence of reactive astrocytes on CNS recovery. This dichotomy can be resolved if the mediators of astrogliosis are identified. We have measured the levels of transcripts encoding inflammatory cytokines in injury systems in which the presence or absence of astrogliosis could be produced selectively. A stab injury to the adult mouse brain using a piece of nitrocellulose (NC) membrane elicited a prompt and marked increase in levels of transcripts for interleukin (IL)-1alpha, IL-1beta, and tumor necrosis factor (TNF)-alpha, which are considered to be microglia/macrophage cytokines. The elevations preceded, or occurred concomitantly with, the rise in glial fibrillary acidic protein mRNA, an early manifestation of astrogliosis. In neonatal mice, IL-1 and TNF-alpha mRNA were elevated to a greater extent by an NC-implant injury, which produced astrogliosis, than after an NC-stab, with minimal astrogliosis. We determined whether endogenous interferon (IFN)-gamma could be responsible for the observed increases in IL-1 and TNF-alpha, because IFN-gamma is a potent microglia/macrophage activator, and because its exogenous administration to rodents enhanced astrogliosis after adult or neonatal insults. A lack of requirement for endogenous IFN-gamma was demonstrated by three lines of evidence. First, no increase in IFN-gamma transcripts could be found at injury. Second, the administration of a neutralizing antibody to IFN-gamma did not attenuate astrogliosis. Third, in IFN-gamma knockout adult mice, astrogliosis and increases in levels of IL-1alpha and TNF-alpha were induced rapidly by injury. The marked elevation of inflammatory cytokines is discussed in the context of astrogliosis and general CNS recovery.

Heterogeneous expression of transketolase in ocular tissues

PURPOSE

Previous studies have shown that transketolase is preferentially expressed in the corneal epithelium and comprises up to 10% of the soluble protein of the mature mouse cornea. The aim of this study is to evaluate the expression and distribution of TKT in the different ocular tissues.

METHODS

We have used in situ hybridization and immunohistochemistry to localize TKT mRNA and protein in the developing and adult mouse eye.

RESULTS

TKT were found to be widely distributed throughout the adult mouse eye. Among the ocular tissues examined, the corneal epithelium exhibited the highest levels of TKT mRNA and protein. Within the epithelial layer, TKT mRNA and protein were differentially distributed with the highest expression occurring in basal cells and the lowest in apical cells, suggesting that TKT expression in the corneal epithelium may be differentiation-related. Enriched expression of TKT was also found in the cornea endothelium, lens epithelium, ciliary body, and iris. Low basal levels of expression were observed in the limbus and conjunctiva. In contrast to the adult eye, TKT expression in the one-day-old mouse eye was homogeneous at low, but detectable levels, suggesting that TKT expression is developmentally regulated in the cornea as well as in the other ocular tissues. In the healing corneal epithelium, TKT expression in the single cell layer of the leading edge was completely suppressed until the cells began to stratify, at which point TKT expression increased markedly.

CONCLUSIONS

The results presented here suggest that TKT is differentially expressed and developmentally regulated in the various tissues that comprise the eye.

Elevated intracellular calcium concentration increases secretory processing of the amyloid precursor protein by a tyrosine phosphorylation-dependent mechanism

Secretory cleavage of the amyloid precursor protein (APP), a process that releases soluble APP derivatives (APPs) into the extracellular space, is stimulated by the activation of muscarinic receptors coupled to phosphoinositide hydrolysis. The signalling pathways involved in the release process exhibit both protein kinase C- and protein tyrosine phosphorylation-dependent components [Slack, Breu, Petryniak, Srivastava and Wurtman (1995) J. Biol. Chem. 270, 8337-8344]. The possibility that elevations in intracellular Ca2+ concentration initiate the tyrosine phosphorylation-dependent release of APPs was examined in human embryonic kidney cells expressing muscarinic m3 receptors. Inhibition of protein kinase C with the bisindolylmaleimide GF 109203X decreased the carbachol-evoked release of APPs by approx. 30%, as shown previously. The residual response was further decreased, in an additive manner, by the Ca2+ chelator EGTA, or by the tyrosine kinase inhibitor tyrphostin A25. The Ca2+ ionophore, ionomycin, like carbachol, stimulated both the release of APPs and the tyrosine phosphorylation of several proteins, one of which was identified as paxillin, a component of focal adhesions. The effects of ionomycin on APPs release and on protein tyrosine phosphorylation were concentration-dependent, and occurred over similar concentration ranges; both effects were inhibited only partly by GF 109203X, but were abolished by EGTA or by tyrosine kinase inhibitors. The results demonstrate for the first time that ionophore-induced elevations in intracellular Ca2+ levels elicit APPs release via increased tyrosine phosphorylation. Part of the increase in APPs release evoked by muscarinic receptor activation might be attributable to a similar mechanism.