Synaptotrophic effects of human amyloid beta protein precursors in the cortex of transgenic mice

Potentiation of excitotoxicity in transgenic mice overexpressing neuronal cyclooxygenase-2

In this study we describe the generation of a transgenic mouse model with neuronal overexpression of the human cyclooxygenase-2, h(COX)-2, to explore its role in excitotoxicity. We report that overexpression of neuronal hCOX-2 potentiates the intensity and lethality of kainic acid excitotoxicity in coincidence with potentiation of expression of the immediate early genes c-fos and zif-268. In vitro studies extended the in vivo findings and revealed that glutamate excitotoxicity is potentiated in primary cortico-hippocampal neurons derived from hCOX-2 transgenic mice, possibly through potentiation of mitochondrial impairment. This study is the first to demonstrate a cause-effect relationship between neuronal COX-2 expression and excitotoxicity. This model system will allow the systematic examination of the role of COX-2 in mechanisms of neurodegeneration that involve excitatory amino acid pathways.

Secreted form of amyloid precursor protein enhances basal glucose and glutamate transport and protects against oxidative impairment of glucose and glutamate transport in synaptosomes by a cyclic GMP-mediated mechanism

Synaptic dysfunction and degeneration are believed to underlie the cognitive deficits that characterize Alzheimer's disease, and overactivation of glutamate receptors under conditions of increased oxidative stress and metabolic compromise may contribute to the neurodegenerative process in many different disorders. The secreted form of amyloid precursor protein (sAPPalpha), which is released from neurons in an activity-dependent manner, can modulate neurite outgrowth, synaptic plasticity, and neuron survival. We now report that sAPPalpha can enhance glucose and glutamate transport in synaptic compartments. Treatment of cortical synaptosomes with nanomolar concentrations of sAPPalpha resulted in an attenuation of impairment of glutamate and glucose transport induced by exposure to amyloid beta-peptide and Fe2+. The protective effect of sAPPalpha was mimicked by treatment with 8-bromo-cyclic GMP and blocked by a cyclic GMP-dependent protein kinase inhibitor, suggesting that protective action of sAPPalpha is mediated by cyclic GMP. Our data suggest that glucose and glutamate transport can be regulated locally at the level of the synapse and further suggest important roles for sAPPalpha and cyclic GMP in modulating synaptic physiology under normal and pathophysiological conditions.

Expression of human apolipoprotein E3 or E4 in the brains of Apoe-/- mice: isoform-specific effects on neurodegeneration

Apolipoprotein (apo) E isoforms are key determinants of susceptibility to Alzheimer's disease. The apoE4 isoform is the major known genetic risk factor for this disease and is also associated with poor outcome after acute head trauma or stroke. To test the hypothesis that apoE3, but not apoE4, protects against age-related and excitotoxin-induced neurodegeneration, we analyzed apoE knockout (Apoe-/-) mice expressing similar levels of human apoE3 or apoE4 in the brain under control of the neuron-specific enolase promoter. Neuronal apoE expression was widespread in the brains of these mice. Kainic acid-challenged wild-type or Apoe-/- mice had a significant loss of synaptophysin-positive presynaptic terminals and microtubule-associated protein 2-positive neuronal dendrites in the neocortex and hippocampus, and a disruption of neurofilament-positive axons in the hippocampus. Expression of apoE3, but not of apoE4, protected against this excitotoxin-induced neuronal damage. ApoE3, but not apoE4, also protected against the age-dependent neurodegeneration seen in Apoe-/- mice. These differences in the effects of apoE isoforms on neuronal integrity may relate to the increased risk of Alzheimer's disease and to the poor outcome after head trauma and stroke associated with apoE4 in humans.

Transgenic animals in Alzheimer's disease research

Alzheimer's disease (AD) is a neurodegenerative disorder of the brain accounting for about 50-70% of the typical late onset cases of dementia. The pathological and diagnostic hallmarks of the disease are principally the presence of extracellular deposits called neuritic amyloid plaques and the intracellular aggregation of neurofibrillary tangles. In addition selective neuronal cell loss accompanied by cerebrovascular amyloidosis is detectable. In the case of familial AD, defects in at least three different genes (APP, PS1, PS2) leading to indistinguishable pathology are now well defined. There is as yet no real treatment for AD. Therefore the availability of an easily manipulable animal model is crucial for the development of new drugs, which could slow down or, even better, stop the progression of the disease. The development and originality of such experimental models that could greatly facilitate the investigation of the aetiology and pathogenesis of AD are described and discussed in this review. They are based mainly on the attempt to reproduce the neurofibrillary tangles or the amyloid deposits and plaque formation.

Amino-terminal region of secreted form of amyloid precursor protein stimulates proliferation of neural stem cells

Beta-amyloid precursor protein (APP) has been reported to be expressed in the CNS from the early stages of development. However, the functional role of APP during early development remains unclear. In the present study, we found that the secreted form of APP (sAPP) significantly enhanced proliferation of neural stem cells. Cells were prepared from 13-day embryonic rat neocortex, which was dissected with a Pasteur pipette to make cell clusters. After 12 h of cultivation in the medium without serum, cells around the centre of the cluster were still nestin-positive proliferative cells, i.e. neural stem cells. To determine whether the proliferation of cells was regulated by sAPP, cultures were treated with recombinant sAPP695, the secreted form of human APP695 produced by yeast. Both DNA synthesis and expression of proliferating cell nuclear antigen markedly increased after 5 h of sAPP695 addition. The enhancement of DNA synthesis by sAPP695 stimulation was blocked by the 22C11 monoclonal antibody specific for the amino-terminal region of sAPP. Then, we examined the effect of the amino-terminal fragment of sAPP and the epitope peptide of 22C11 antibody, and found that both of them also promoted DNA synthesis, suggesting that the amino-terminal region of sAPP is responsible for the biological activity. Our findings indicate the possibility that sAPP enhances proliferation of neural stem cells in vivo and plays an important role during the early CNS development.

No hippocampal neuron or synaptic bouton loss in learning-impaired aged beta-amyloid precursor protein-null mice

Aged beta-amyloid precursor protein-null mice were used to investigate the relationship between beta-amyloid precursor protein, hippocampal neuron and synaptic bouton number, and cognitive function. Learning and memory performance of aged beta-amyloid precursor protein-null mice and age-matched controls were assessed in the Morris water maze. Beta-amyloid precursor protein-null mice demonstrated impaired task acquisition as measured by significantly longer swim path lengths, a higher percentage of failed trials, and more frequent thigmotaxis behavior than controls. In a subsequent probe trial, beta-amyloid precursor protein-null mice spent significantly less time in the old goal quadrant, and made fewer crossings over the old platform location than did controls. No differences in motor or visual skills were observed which could account for the performance differences. In light of these findings and previous evidence for a role of beta-amyloid precursor protein in neuronal maintenance and synaptogenesis, we pursued the hypothesis that the learning impairment of beta-amyloid precursor protein-null mice may be a reflection of differences in neuron or synaptophysin-positive presynaptic bouton number. Thus, unbiased stereological analysis was used to estimate neuron and synaptic bouton number in dentate gyrus and hippocampal CA1 of the behaviorally characterized mice. No difference in neuron or synaptophysin-positive presynaptic bouton number was found between the beta-amyloid precursor protein-null mice and age-matched controls. Our results suggest that the learning impairment of beta-amyloid precursor protein-null mice is not mediated by a loss of hippocampal neurons or synaptic boutons.

Reorganization of cholinergic terminals in the cerebral cortex and hippocampus in transgenic mice carrying mutated presenilin-1 and amyloid precursor protein transgenes

Cholinergic deficits are one of the most consistent neuropathological landmarks in Alzheimer's disease (AD). We have examined transgenic mouse models (PS1M146L, APPK670N,M671L) and a doubly transgenic line (APPK670N,M671L + PS1M146L) that overexpress mutated AD-related genes [presenilin-1 (PS1) and the amyloid precursor protein (APP)] to investigate the effect of AD-related gene overexpression and/or amyloidosis on cholinergic parameters. The size of the basal forebrain cholinergic neurons and the pattern of cholinergic synapses in the hippocampus and cerebral cortex were revealed by immunohistochemical staining for choline acetyltransferase and the vesicular acetylcholine transporter, respectively. At the time point studied (8 months), no apparent changes in either the size or density of cholinergic synapses were found in the PS1M146L mutant relative to the nontransgenic controls. However, the APPK670N,M671L mutant showed a significant elevation in the density of cholinergic synapses in the frontal and parietal cortices. Most importantly, the double mutant (APPK670N,M671L + PS1M146L), which had extensive amyloidosis, demonstrated a prominent diminution in the density of cholinergic synapses in the frontal cortex and a reduction in the size of these synapses in the frontal cortex and hippocampus. Nonetheless, no significant changes in the size of basal forebrain cholinergic neurons were observed in these three mutants. This study shows a novel role of APP and a synergistic effect of APP and PS1 that correlates with amyloid load on the reorganization of the cholinergic network in the cerebral cortex and hippocampus at the time point studied.

The amyloid precursor protein of Alzheimer's disease and the Abeta peptide

Alzheimer's disease is characterized by the accumulation of beta amyloid peptides in plaques and vessel walls and by the intraneuronal accumulation of paired helical filaments composed of hyperphosphorylated tau. In this review, we concentrate on the biology of amyloid precursor protein, and on the central role of amyloid in the pathogenesis of Alzheimer's disease. Amyloid precursor protein (APP) is part of a super-family of transmembrane and secreted proteins. It appears to have a number of roles, including regulation of haemostasis and mediation of neuroprotection. APP also has potentially important metal and heparin-binding properties, and the current challenge is to synthesize all these varied activities into a coherent view of its function. Cleavage of amyloid precursor protein by beta-and gamma-secretases results in the generation of the Abeta (betaA4) peptide, whereas alpha-secretase cleaves within the Abeta sequence and prevents formation from APP. Recent findings indicate that the site of gamma-secretase cleavage is critical to the development of amyloid deposits; Abeta1-42 is much more amyloidogenic than Abeta1-40. Abeta1-42 formation is favoured by mutations in the two presenilin genes (PS1 and PS2), and by the commonest amyloid precursor protein mutations. Transgenic mouse models of Alzheimer's disease incorporating various mutations in the presenilin gene now exist, and have shown amyloid accumulation and cognitive impairment. Neurofibrillary tangles have not been reproduced in these models, however. While aggregated Abeta is neurotoxic, perhaps via an oxidative mechanism, the relationship between such toxicity and neurofibrillary tangle formation remains a subject of ongoing research.

Age-related cognitive deficits, impaired long-term potentiation and reduction in synaptic marker density in mice lacking the beta-amyloid precursor protein

Mutations in the beta-amyloid precursor protein are strongly associated with some cases of familial Alzheimer's disease. The normal physiological role of beta-amyloid precursor protein in the brain was evaluated in a cross-sectional analysis of mice deficient in beta-amyloid precursor protein. Compared with wild-type control mice the beta-amyloid precursor protein-null mice developed age-dependent deficits in cognitive function and also had impairments in long-term potentiation. In addition, the brains of the beta-amyloid precursor protein-null mice had marked reactive gliosis in many areas, especially in the cortex and hippocampus. A subpopulation of mice (n = 15) died prematurely (between three and 18 months of age). Analysis of another six mice from the same population that were showing weight loss and hypolocomotor activity exhibited a marked reactive gliosis as detected by immunoreactivity for glial fibrillary acidic protein and a profound loss of immunoreactivities for the presynaptic terminal vesicle marker proteins synaptophysin and synapsin and the dendritic marker microtubule-associated protein-2 in many brain areas, but most predominantly in the cortex and hippocampus. These results suggest that normal beta-amyloid precursor protein may serve an essential role in the maintenance of synaptic function during ageing. A compromise of this function of the beta-amyloid precursor protein may contribute to the progression of the memory decline and the neurodegenerative changes seen in Alzheimer's disease.

Early phenotypic changes in transgenic mice that overexpress different mutants of amyloid precursor protein in brain

Transgenic mice overexpressing different forms of amyloid precursor protein (APP), i.e. wild type or clinical mutants, displayed an essentially comparable early phenotype in terms of behavior, differential glutamatergic responses, deficits in maintenance of long term potentiation, and premature death. The cognitive impairment, demonstrated in F1 hybrids of the different APP transgenic lines, was significantly different from nontransgenic littermates as early as 3 months of age. Biochemical analysis of secreted and membrane-bound APP, C-terminal "stubs," and Abeta(40) and Abeta(42) peptides in brain indicated that no single intermediate can be responsible for the complex of phenotypic dysfunctions. As expected, the Abeta(42) levels were most prominent in APP/London transgenic mice and correlated directly with the formation of amyloid plaques in older mice of this line. Plaques were associated with immunoreactivity for hyperphosphorylated tau, eventually signaling some form of tau pathology. In conclusion, the different APP transgenic mouse lines studied display cognitive deficits and phenotypic traits early in life that dissociated in time from the formation of amyloid plaques and will be good models for both early and late neuropathological and clinical aspects of Alzheimer's disease.

Progress toward valid transgenic mouse models for Alzheimer's disease

A transgenic mouse model for Alzheimer's disease (AD) should mimic the age-dependent accumulation of beta-amyloid plaques, neurofibrillary tangles, neuronal cell death as well as display memory loss and behavioral deficits. Age-dependent accumulation of A beta deposits in mouse brain has been achieved in mice overexpressing mutant alleles of the amyloid precursor protein (APP). In contrast, mice bearing mutant alleles of the presenilin genes show increased production of the A beta42 peptide, but do not form amyloid deposits unless mutant alleles of APP are also overproduced. Furthermore, the onset of A beta deposition is greatly accelerated, paralleling the involvement of presenilins in early onset AD. Studies of APP and presenilin transgenic mice have shown 1) the absence of a requirement for a maturation step in dense core plaque formation, 2) evidence that beta-amyloid deposition is directed by regional factors, and 3) behavioral deficits are observed before A beta deposition. Crosses of APP transgenic mice with mice modified for known AD risk factors and "humanizing" the mouse may be necessary for complete replication of AD.

Alzheimer's disease and oxidative stress: implications for novel therapeutic approaches

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with a deadly outcome. AD is the leading cause of senile dementia and although the pathogenesis of this disorder is not known, various hypotheses have been developed based on experimental data accumulated since the initial description of this disease by Alois Alzheimer about 90 years ago. Most approaches to explain the pathogenesis of AD focus on its two histopathological hallmarks, the amyloid beta protein- (A(beta)-) loaded senile plaques and the neurofibrillary tangles, which consist of the filament protein tau. Various lines of genetic evidence support a central role of A(beta) in the pathogenesis of AD and an increasing number of studies show that oxidation reactions occur in AD and that A(beta) may be one molecular link between oxidative stress and AD-associated neuronal cell death. A(beta) itself can be neurotoxic and can induce oxidative stress in cultivated neurons. A(beta) is, therefore, one player in the concert of oxidative reactions that challenge neurons besides inflammatory reactions which are also associated with the AD pathology. Consequently, antioxidant approaches for the prevention and therapy of AD are of central interest. Experimental as well as clinical data show that lipophilic antioxidants, such as vitamin E and estrogens, are neuroprotective and may help patients suffering from AD. While an additional intensive elucidation of the cellular and molecular events of neuronal cell death in AD will, ultimately, lead to novel drug targets, various antioxidants are already available for a further exploitation of their preventive and therapeutic potential. reserved

Myeloperoxidase polymorphism is associated with gender specific risk for Alzheimer's disease

Myeloperoxidase (MPO) is a myeloid-specific enzyme that generates hypochlorous acid and other reactive oxygen species. MPO is present at high levels in circulating neutrophils and monocytes but is not detectable in microglia, brain-specific macrophages, in normal brain tissue. However, an earlier study indicated that MPO is present in macrophage-microglia at multiple sclerosis lesions, suggesting that reactivation of MPO gene expression may play a role in neurodegenerative diseases involving macrophage-microglia. In the present study, MPO is shown to colocalize with amyloid beta (Abeta) in senile plaques in cerebral cortex sections from Alzheimer's disease (AD) brain tissue. Microglia costaining for MPO and CD68 are closely associated with plaques, suggesting that plaque components induce MPO expression in microglia. In support of this interpretation, treatment of rodent microglia with aggregated Abeta(1-42) was shown to induce MPO mRNA expression. Also, the ApoE4 allele, the major AD risk factor associated with increased Abeta deposition, was shown to correlate with increased MPO deposition in plaques (P = 0.01, ANOVA). Finally, a genetic polymorphism links MPO expression to Alzheimer's risk, in that a higher expressing SpSp MPO genotype was associated with increased incidence of AD in females, and decreased incidence in males (P = 0.006). These findings suggest that the MPO polymorphism is a gender-specific risk factor for Alzheimer's disease.

Long-term basal forebrain cholinergic-rich grafts derived from trisomy 16 mice do not develop beta-amyloid pathology and neurodegeneration but demonstrate neuroinflammatory responses

Patients with Down syndrome (human trisomy 21) develop neuropathological and cholinergic functional defects characteristic of Alzheimer's disease, which has been attributed to the location of the Alzheimer beta-amyloid precursor protein on chromosome 21. Due to the partial genetic homology between mouse chromosome 16 and human chromosome 21, murine trisomy 16 was used as a model to study functional links between increased expression of the amyloid precursor protein, neurodegeneration and neuroinflammatory responses. Basal forebrain cholinergic-rich tissue derived from trisomy 16 mice at embryonic age of day 16 was transplanted into the lateral ventricle of adult normal mice. At 1, 3, 6, 9 and 12 months after transplantation, the grafts were characterized by immunocytochemistry, molecular biological analysis, and stereological methods. Grafts survived up to one year and still demonstrated immunoreactivity for cholinergic, GABAergic and astroglial cells. Though a 1.5-fold neuronal over-expression of amyloid precursor protein was detected in brains from trisomy 16 embryos by Northern analysis, beta-amyloid deposits were found neither in control nor trisomic grafts. Detailed stereological analysis of trisomic grafts did not reveal any neurodegeneration or morphological changes of cholinergic and GABAergic neurons during the course of graft maturation up to one year, as compared to grafts derived from euploid tissue. However, both euploid and trisomic grafts demonstrated a strong infiltration with T- and B-lymphocytes and a significant micro- and astroglial activation (hypertrophic astrocytes) within and around the grafts. These observations further suggest that the trisomy 16-induced neurodegeneration is seemingly due to a lack of neuron supporting factors which are provided by either the metabolic interaction of trisomic graft with surrounding healthy host tissue or by cells of the immune system infiltrating the graft.

Transgenic mice expressing an alpha-secretion mutant of the amyloid precursor protein in the brain develop a progressive CNS disorder

Expression of alpha-secretion mutant APP/RK in mouse brain results in a progressive disorganization of the central nervous system, exemplified by behavioral deficits, premature death and neuropathology. Here we report on the progressive nature of this CNS disorder as indicated by the age dependency of the neophobic reaction in the open-field test. The earlier reported NMDA hypo-sensitivity in the transgenic APP/RK mice is likely to represent a subtle functional disturbance, since no changes in NMDA receptor density or distribution could be detected. None of the typical neuropathological hallmarks of Alzheimer's Disease, i.e. amyloid deposits and neurofibrillary tangles are detected in the brain of these transgenic mice. Nevertheless, the progressive CNS disorder elicited in the transgenic APP/RK mice recapitulates certain features and symptoms of patients with Alzheimer's disease as discussed.

Mechanisms of synaptic pathology in Alzheimer's disease

Neurodegenerative disorders are characterized by damage to selective neuronal populations that could be followed or preceded by synaptic injury. Therefore, specific mutations in and other alterations of synaptic proteins might lead to particular neurodegenerative diseases. The predominant hypothesis is that these mutations result in an increased production of amyloid beta-protein 1-42 which acts as a neurotoxin. However, it could also be postulated that amyloid precursor protein might play an important role in synaptic function and neuronal maintenance and that its abnormal activity may lead to neurodegeneration. Recent studies have shown that amyloid precursor protein has an important role in regulating glutamate levels at the synaptic site by modulating the activity of glutamate transporters. The objectives of this manuscript are to highlight recent data supporting the hypothesis that neurodegeneration in Alzheimer's disease might be the combined result of abnormal protective activity of amyloid precursor protein and amyloid beta-protein toxicity.

Amyloid protein precursor stimulates excitatory amino acid transport. Implications for roles in neuroprotection and pathogenesis

Excitatory neurotransmitters such as glutamate are required for the normal functioning of the central nervous system but can trigger excitotoxic neuronal injury if allowed to accumulate to abnormally high levels. Their extracellular levels are controlled primarily by transmitter uptake into astrocytes. Here, we demonstrate that the amyloid protein precursor may participate in the regulation of this important process. The amyloid protein precursor has been well conserved through evolution, and a number of studies indicate that it may function as an endogenous excitoprotectant. However, the mechanisms underlying this neuroprotective capacity remain largely unknown. At moderate levels of expression, human amyloid protein precursors increased glutamate/aspartate uptake in brains of transgenic mice, with the 751-amino acid isoform showing greater potency than the 695-amino acid isoform. Cerebral glutamate/aspartate transporter protein levels were higher in transgenic mice than in non-transgenic controls, whereas transporter mRNA levels were unchanged. Amyloid protein precursor-dependent stimulation of aspartate uptake by cultured primary astrocytes was associated with increases in protein kinase A and C activity and could be blocked by inhibitors of these kinases. The stimulation of astroglial excitatory amino acid transport by amyloid protein precursors could protect the brain against excitotoxicity and may play an important role in neurotransmission.

GABAergic deafferentation hypothesis of brain aging and Alzheimer's disease revisited

Considering the mechanisms responsible for age- and Alzheimer's disease (AD)-related neuronal degeneration, little attention was paid to the opposing relationships between the energy-rich phosphates, mainly the availability of the adenosine triphosphate (ATP), and the activity of the glutamic acid decarboxylase (GAD), the rate-limiting enzyme synthesizing the gamma-amino butyric acid (GABA). Here, it is postulated that in all neuronal phenotypes the declining ATP-mediated negative control of GABA synthesis gradually declines and results in age- and AD-related increases of GABA synthesis. The Ca2+-independent carrier-mediated GABA release interferes with Ca2+-dependent exocytotic release of all transmitter-modulators, because the interstitial (ambient) GABA acts on axonal preterminal and terminal varicosities endowed with depolarizing GABA(A)-benzodiazepine receptors; this makes GABA the "executor" of virtually all age- and AD-related neurodegenerative processes. Such a role of GABA is diametrically opposite to that in the perinatal phase, when the carrier-mediated GABA release, acting on GABA(A)/chloride ionophore receptors, positively controls chemotactic migration of neuronal precursor cells, has trophic actions and initiates synaptogenesis, thereby enabling retrograde axonal transport of target produced factors that trigger differentiation of neuronal phenotypes. However, with advancing age, and prematurely in AD, the declining mitochondrial ATP synthesis unleashes GABA synthesis, and its carrier-mediated release blocks Ca2+-dependent exocytotic release of all transmitter-modulators, leading to dystrophy of chronically depolarized axon terminals and block of retrograde transport of target-produced trophins, causing "starvation" and death of neuronal somata. The above scenario is consistent with the following observations: 1) a 10-month daily administration to aging rats of the GABA-chloride ionophore antagonist, pentylenetetrazol, or of the BDZ antagonist, flumazenil (FL), each forestalls the age-related decline in cognitive functions and losses of hippocampal neurons; 2) the brains of aging rats, relative to young animals, and the postmortem brains of AD patients, relative to age-matched controls, show up to two-fold increases in GABA synthesis; 3) the aging humans and those showing symptoms of AD, as well as the aging nonhuman primates and rodents--all show in the forebrain dystrophic axonal varicosities, losses of transmitter vesicles, and swollen mitochondria. These markers, currently regarded as the earliest signs of aging and AD, can be reproduced in vitro cell cultures by 1 microM GABA; the development of these markers can be prevented by substituting Cl- with SO4(2-); 4) the extrasynaptic GABA suppresses the membrane Na+, K+-ATPase and ion pumping, while the resulting depolarization of soma-dendrites relieves the "protective" voltage-dependent Mg2+ control of the N-methyl-D-aspartate (NMDA) channels, thereby enabling Ca2+-dependent persistent toxic actions of the excitatory amino acids (EAA); and 5) in whole-cell patch-clamp recording from neurons of aging rats, relative to young rats, the application of 3 microM GABA, causes twofold increases in the whole-cell membrane Cl- conductances and a loss of the physiologically important neuronal ability to desensitize to repeated GABA applications. These age-related alterations in neuronal membrane functions are amplified by 150% in the presence of agonists of BDZ recognition sites located on GABA receptor. The GABA deafferentation hypothesis also accounts for the age- and AD-related degeneration in the forebrain ascending cholinergic, glutamatergic, and the ascending mesencephalic monoaminergic system, despite that the latter, to foster the distribution-utilization of locally produced trophins, evolved syncytium-like connectivities among neuronal somata, axon collaterals, and dendrites, to bidirectionally transport trophins. (ABSTRACT TRUNCATED)

Enhanced GFAP expression in astrocytes of transgenic mice expressing the human brain-specific trypsinogen IV

We recently identified a cDNA encoding a human brain specific trypsinogen (trypsinogen IV). In order to test whether trypsinogen IV is involved in CNS diseases of, or injury response in, mammalian brain, a mouse model was developed in which the human trypsinogen IV was expressed specifically in neurons. Immunocytochemical analysis of the brains of transgenic mice revealed a striking enhancement of glial fibrillar acidic protein (GFAP) expression in astrocytes. This remarkable astrocytic reaction was detected in the brains of mice as young as 2 months and did not diminish in the older animals we tested. However, we did not find gross evidence for neurodegeneration, nor for reactive microglial cells. The long-term survival of these animals should provide a model with which to study the mechanism of nerve-astroglia interactions. In addition, the possible participation of trypsin IV in the metabolism of the Alzheimer precursor protein (APP) was investigated by immunostaining brains from transgenic mice with beta-amyloid (betaA4) antibodies. Immunocytochemical staining of brains from one year old transgenic mice revealed an intense intracellular betaA4-like signal in neurons.

Twofold overexpression of human beta-amyloid precursor proteins in transgenic mice does not affect the neuromotor, cognitive, or neurodegenerative sequelae following experimental brain injury

By using transgenic mice that overexpress human beta-amyloid precursor proteins (APPs) at levels twofold higher than endogenous APPs, following introduction of the human APP gene in a yeast artificial chromosome (YAC), we examined the effects of controlled cortical impact (CCI) brain injury on neuromotor/cognitive dysfunction and the development of Alzheimer's disease (AD)-like neuropathology. Neuropathological analyses included Nissl-staining and immunohistochemistry to detect APPs, beta-amyloid (Abeta), neurofilament proteins, and glial fibrillary acidic protein, whereas Abeta levels were measured in brain homogenates from mice subjected to CCI and control mice by using a sensitive sandwich enzyme-linked immunosorbent assay. Twenty APP-YAC transgenic mice and 17 wild type (WT) littermate controls were anesthetized and subjected to CCI (velocity, 5 m/second; deformation depth, 1 mm). Sham (anesthetized but uninjured) controls (n = 10 APP-YAC; n = 8 WT) also were studied. Motor function was evaluated by using rotarod, inclined-plane, and forelimb/hindlimb flexion tests. The Morris water maze was used to assess memory. Although CCI induced significant motor dysfunction and cognitive deficits, no differences were observed between brain-injured APP-YAC mice and WT mice at 24 hours and 1 week postinjury. By 1 week postinjury, both cortical and hippocampal CA3 neuron loss as well as extensive astrogliosis were observed in all injured animals, suggesting that overexpression of human APPs exhibited no neuroprotective effects. Although AD-like pathology (including amyloid plaques) was not observed in either sham or brain-inj ured animals, a significant decrease in brain concentrations of only Abeta terminating at amino acid 40 (Abeta x-40) was observed following brain injury in APP-YAC mice (P < 0.05 compared with sham control levels). Our data show that the APP-YAC mice do not develop AD-like neuropathology following traumatic brain injury. This may be because this injury does not induce elevated levels of the more amyloidogenic forms of human Abeta (i.e., Abeta x-42/43) in these mice.

Manipulation of gene expression in the mammalian nervous system: application in the study of neurite outgrowth and neuroregeneration-related proteins

A fundamental issue in neurobiology entails the study of the formation of neuronal connections and their potential to regenerate following injury. In recent years, an expanding number of gene families has been identified involved in different aspects of neurite outgrowth and regeneration. These include neurotrophic factors, cell-adhesion molecules, growth-associated proteins, cytoskeletal proteins and chemorepulsive proteins. Genetic manipulation technology (transgenic mice, knockout mice, viral vectors and antisense oligonucleotides) has been instrumental in defining the function of these neurite outgrowth-related proteins. The aim of this paper is to provide an overview of the above-mentioned four approaches to manipulate gene expression in vivo and to discuss the progress that has been made using this technology in helping to understand the molecular mechanisms that regulate neurite outgrowth. We will show that work with transgenic mice and knockout mice has contributed significantly to the dissection of the function of several proteins with a key role in neurite outgrowth and neuronal survival. Recently developed viral vectors for gene transfer in postmitotic neurons have opened up new avenues to analyze the function of a protein following local expression in naive adult rodents. The initial results with viral vector-based gene transfer provide a conceptual framework for further studies on genetic therapy of neuroregeneration and neurodegenerative diseases.

Secreted amyloid precursor protein alpha selectively suppresses N-methyl-D-aspartate currents in hippocampal neurons: involvement of cyclic GMP

The secreted form of beta-amyloid precursor protein (sAPP alpha) is released from neurons in an activity-dependent manner; data suggest sAPP alpha may play roles in regulating neuronal excitability, plasticity, and survival. In cultured hippocampal neurons sAPP alpha can suppress elevation of [Ca2+]i induced by glutamate and can protect neurons against excitotoxicity. We now report whole-cell patch-clamp data from studies of cultured embryonic rat hippocampal neurons which demonstrate that sAPP alpha selectively suppresses N-methyl-D-aspartate currents without affecting currents induced by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate or kainate. sAPP alpha suppressed N-methyl-D-aspartate current rapidly and reversibly at concentrations of 0.011 nM. Suppression of N-methyl-D-aspartate current by sAPP alpha is apparently mediated by cyclic guanosine monophosphate because 8-bromo-cyclic guanosine monophosphate suppressed N-methyl-D-aspartate current in a manner similar to sAPP alpha, and two different inhibitors of cyclic guanosine monophosphate-dependent protein kinase prevented sAPP alpha-induced suppression of N-methyl-D-aspartate current. In addition, okadaic acid prevented suppression of N-methyl-D-aspartate-induced current suggesting the involvement of a protein phosphatase in modulation of N-methyl-D-aspartate current by sAPP alpha. These data identify a mechanism whereby sAPP alpha can modulate cellular responses to glutamate, and suggest important roles for sAPP alpha in the various physiological and pathophysiological processes in which N-methyl-D-aspartate receptors participate.

Involvement of amyloid precursor protein in functional synapse formation in cultured hippocampal neurons

Amyloid precursor protein (APP) is known to be widely expressed in neuronal cells, and enriched in the central and peripheral synaptic sites. Although it has been proposed that APP functions in synaptogenesis, no direct evidence has yet been reported. In this study we investigated the involvement of APP in functional synapse formation by monitoring spontaneous oscillations of intracellular Ca2+ concentration ([Ca2+]i) in cultured hippocampal neurons. As more and more neurons form synapses with each other during the culture period, increasing numbers of neuronal cells show synchronized spontaneous oscillations of [Ca2+]i. The number of neurons that showed synchronized spontaneous oscillations of [Ca2+]i was significantly lower when cultured in the presence of monoclonal antibody 22C11 against the N-terminal portion of APP. Moreover, incubation with excess amounts of the secretory form of APP or the N-terminal fragment of APP also inhibited the increase in number of neurons with synchronized spontaneous oscillations of [Ca2+]i. The addition of monoclonal antibody 22C11 or secretory form of APP did not, however, affect MAP-2-positive neurite outgrowth. These findings suggest that APP play a role in functional synapse formation during CNS development.

Two amyloid precursor protein transgenic mouse models with Alzheimer disease-like pathology

Mutations in the amyloid precursor protein (APP) gene cause early-onset familial Alzheimer disease (AD) by affecting the formation of the amyloid beta (A beta) peptide, the major constituent of AD plaques. We expressed human APP751 containing these mutations in the brains of transgenic mice. Two transgenic mouse lines develop pathological features reminiscent of AD. The degree of pathology depends on expression levels and specific mutations. A 2-fold overexpression of human APP with the Swedish double mutation at positions 670/671 combined with the V717I mutation causes A beta deposition in neocortex and hippocampus of 18-month-old transgenic mice. The deposits are mostly of the diffuse type; however, some congophilic plaques can be detected. In mice with 7-fold overexpression of human APP harboring the Swedish mutation alone, typical plaques appear at 6 months, which increase with age and are Congo Red-positive at first detection. These congophilic plaques are accompanied by neuritic changes and dystrophic cholinergic fibers. Furthermore, inflammatory processes indicated by a massive glial reaction are apparent. Most notably, plaques are immunoreactive for hyperphosphorylated tau, reminiscent of early tau pathology. The immunoreactivity is exclusively found in congophilic senile plaques of both lines. In the higher expressing line, elevated tau phosphorylation can be demonstrated biochemically in 6-month-old animals and increases with age. These mice resemble major features of AD pathology and suggest a central role of A beta in the pathogenesis of the disease.

Pathogenesis of decreased glucose turnover and oxidative phosphorylation in ischemic and trauma-induced dementia of the Alzheimer type

The pathogenetic mechanisms causing a dementing brain disease after temporary ischemia, heat shock, or brain trauma are surveyed. These lesions increase beta amyloid precursor protein (beta APP) synthesis. This process is potentiated by an ischemic glutamate release that opens cellular Ca2+ channels, inhibiting glucose turnover and ATP production, which is, under these conditions, accompanied by the generation of beta amyloid (beta A), even in young persons. Beta amyloid starts a vicious circle by inactivating the glycolytic key enzyme, phosphofructokinase, which, with age, exhausts the functional reserve capacity of the brain. This demonstrates that beta A is an epiphenomenon of a dementing brain disease, triggered by the disturbance of glucose turnover and oxidative phosphorylation. Clinical studies have shown that a dementing brain disease can be clearly objectified and monitored by 18F-2-deoxyglucose PET studies. This paper looks briefly at pharmacologic approaches to this disease using models of temporary ischemia, the testing of 14C-deoxyglucose turnover, or examination with 31P magnetic resonance spectroscopy techniques. In conclusion, the key process of all dementing brain diseases of the Alzheimer type is a decreased glucose turnover and subsequently decreased oxidative phosphorylation, linked directly to a secondary amyloid formation and nerve cell atrophy.

Synaptic beta-amyloid precursor proteins increase with learning capacity in rats

The precursor proteins of Alzheimer's disease beta-amyloid peptide, the beta-amyloid precursor protein isoforms, comprise a family of neuronal proteins with synaptic localization whose physiological roles in brain are poorly understood. One possible role for synaptic proteins is involvement in neuronal plasticity. After exposure to an enriched environment compared to impoverished conditions, rats exhibited superior cognitive capacity. Up to approximately four-fold increased overall levels of beta-amyloid precursor proteins were found in cortical/subcortical tissue of the enriched animals displaying significantly more synapses immunoreactive for the different beta-amyloid precursor protein isoforms (beta-amyloid precursor protein695- and beta-amyloid precursor protein751/770) in hippocampus and adjacent occipital cortex. This correlation thus provides in vivo evidence for an association of beta-amyloid precursor proteins with plastic changes induced by complex environment with consequences for cognitive functions and suggests that impaired beta-amyloid precursor protein metabolism at synapses might contribute to brain dysfunction in Alzheimer's disease.

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.

Amyloid precursor proteins protect neurons of transgenic mice against acute and chronic excitotoxic injuries in vivo

The beta-amyloid protein precursor (APP) is well conserved across different species and may fulfill important physiological functions within the CNS. While high-level neuronal expression of amyloidogenic forms of human APP results in beta-amyloid production and neurodegeneration, lower levels of neuronal human APP expression in neurons of transgenic mice may primarily accentuate physiological functions of this molecule. To assess the neuroprotective potential of human APP in vivo, mice from seven distinct transgenic lines expressing different human APP isoforms from the neuron-specific enolase promoter were challenged with systemic kainate injections (n=30) or transgene-mediated glial expression of gp120 (n=32), an HIV-1 protein capable of inducing excitotoxic neuronal damage. To quantitate human APP-mediated neuroprotection. the area of neuropil occupied by presynaptic terminals and neuronal dendrites in the neocortex and hippocampus of each mouse was determined using laser scanning confocal microscopy of double-immunolabelled brain sections and computer-aided image analysis. Compared with gp120 singly transgenic controls, mice from three of three human APP751gp120 bigenic lines expressing the 751 amino acid form of human APP at low levels showed significant protection against degeneration of presynaptic terminals; two of these lines also showed significantly less damage to neuronal dendrites. Two of three human APP695/gp120 bigenic lines expressing human APP695 at low levels were protected against presynaptic and dendritic damage, whereas one low expressor line and a human APP695/gp120 bigenic line expressing human APP695 at higher levels showed no significant protection. In the corresponding human APP singly transgenic lines, overexpressing only specific human APP isoforms, significant protection against kainate-induced degeneration of presynaptic terminals and neuronal dendrites was found in two of three human APP751 lines and not in any of the four human APP695 lines tested. These results indicate that human APP can protect neurons against chronic and acute excitotoxic insults in vivo and that human APP isoforms differ in their neuroprotective potential, at least with respect to specific forms of neural injury. It is therefore possible that impairments of neuroprotective human APP functions or aberrant shifts in human APP isoform ratios could contribute to neurodegeneration.

Murine models of brain aging and age-related neurodegenerative diseases

In the past, structural changes in the brain with aging have been studied using a variety of animal models, with rats and nonhuman primates being the most popular. With the rapid evolution of mouse genetics, murine models have gained increased attention in the neurobiology of aging. The genetic contribution of age-related traits as well as specific mechanistic hypotheses underlying brain aging and age-related neurodegenerative diseases can now be assessed by using genetically-selected and genetically-manipulated mice. Against this background of increased demand for aging research in mouse models, relatively few studies have examined structural alterations with aging in the normal mouse brain, and the data available are almost exclusively restricted to the C57BL/6 strain. Moreover, many older studies have used quantitative techniques which today can be questioned regarding their accuracy. Here we review the state of knowledge about structural changes with aging in outbred, inbred, genetically-selected, and genetically-engineered murine models. Moreover, we suggest several new opportunities that are emerging to study brain aging and age-related neurodegenerative diseases using genetically-defined mouse models. By reviewing the literature, it has become clear to us that in light of the rapid progress in genetically-engineered and selected mouse models for brain aging and age-related neurodegenerative diseases, there is a great and urgent need to study and define morphological changes in the aging brain of normal inbred mice and to analyze the structural changes in genetically-engineered mice more carefully and completely than accomplished to date. Such investigations will broaden knowledge in the neurobiology of aging, particularly regarding the genetics of aging, and possibly identify the most useful murine models.

Possible mechanism for resistance to Alzheimer's disease (AD) in mice suggests a new approach to generate a mouse model for sporadic AD and may explain familial resistance to AD in man

An overproduction of beta-amyloid (A beta) is associated with Alzheimer's disease (AD) and appears to be its primary cause. A model has been recently described which accounts for the overproduction of A beta in sporadic AD, this constituting the majority of all cases of AD. The proposed mechanism suggests the antisense RNA-Mediated generation of a 5'-truncated beta-amyloid precursor protein (beta APP) mRNA encoding a 12-kDa C-terminal fragment of beta APP, the immediate precursor of A beta. In the truncated mRNA, the first AUG codon, which contiguously precedes the A beta-coding segment, becomes the site of translation initiation of a polypeptide that can be further processed to generate A beta, this subsequently being secreted. Among the predictions of the proposed model is that mice and rats do not and indeed cannot develop sporadic AD because they lack the crucial component of the proposed mechanism, namely the ability of the beta APP antisense RNA to self-prime the synthesis of a new sense strand. According to the proposed model, however, mice could be rendered susceptible to AD by mutating the beta APP gene so as to confer self-priming ability on the antisense strand. In contrast to existing mouse models which by design are fundamentally unsuitable for study of the mechanism underlying sporadic AD, the AD pathology of the proposed model would be expected to faithfully reflect the human condition. The availability of such an acutely needed, experimental model would allow investigators to study not only the manifestation of the disease but, most significantly, also the factors triggering it. The proposed mouse model may explain familial resistance to AD in man, provide extremely valuable insights into the etiology of AD, and suggest means for its prevention.

Cellular signaling roles of TGF beta, TNF alpha and beta APP in brain injury responses and Alzheimer's disease

beta-Amyloid precursor protein (beta APP), transforming growth factor beta (TGF beta), and tumor necrosis factor-alpha (TNF alpha) are remarkably pleiotropic neural cytokines/neurotrophic factors that orchestrate intricate injury-related cellular and molecular interactions. The links between these three factors include: their responses to injury; their interactive effects on astrocytes, microglia and neurons; their ability to induce cytoprotective responses in neurons; and their association with cytopathological alterations in Alzheimer's disease. Astrocytes and microglia each produce and respond to TGF beta and TNF alpha in characteristic ways when the brain is injured. TGF beta, TNF alpha and secreted forms of beta APP (sAPP) can protect neurons against excitotoxic, metabolic and oxidative insults and may thereby serve neuroprotective roles. On the other hand, under certain conditions TNF alpha and the fibrillogenic amyloid beta-peptide (A beta) derivative of beta APP can promote damage of neuronal and glial cells, and may play roles in neurodegenerative disorders. Studies of genetically manipulated mice in which TGF beta, TNF alpha or beta APP ligand or receptor levels are altered suggest important roles for each factor in cellular responses to brain injury and indicate that mediators of neural injury responses also have the potential to enhance amyloidogenesis and/or to interfere with neuroregeneration if expressed at abnormal levels or modified by strategic point mutations. Recent studies have elucidated signal transduction pathways of TGF beta (serine/threonine kinase cascades), TNF alpha (p55 receptor linked to a sphingomyelin-ceramide-NF kappa B pathway), and secreted forms of beta APP (sAPP; receptor guanylate cyclase-cGMP-cGMP-dependent kinase-K+ channel activation). Knowledge of these signaling pathways is revealing novel molecular targets on which to focus neuroprotective therapeutic strategies in disorders ranging from stroke to Alzheimer's disease.

Expression of amyloid precursor protein mRNA isoforms in rat brain is differentially regulated during postnatal maturation and by cholinergic activity

Pathological processing of the amyloid precursor protein (APP) is assumed to be responsible for the amyloid deposits in Alzheimer-diseased brain tissue, but the physiological function of this protein in the brain is still unclear. The aim of this study is to reveal whether the expression of different splicing variants of APP transcripts in distinct brain regions is driven by postnatal maturation and/or regulated by cortical cholinergic transmission, applying quantitative in situ hybridization histochemistry using 35S-labeled oligonucleotides as specific probes to differentiate between APP isoforms. In cortical brain regions, the expression of both APP695 and APP751 is high at birth and exhibits nearly adult levels. The developmental expression pattern of cortical APP695 displays a peak value around postnatal day 10, while the age-related expression of APP751 demonstrates peak values on postnatal days 10 and 25, with the highest steady state levels of APP751 mRNA on day 25. During early development, the cortical laminar distribution of the APP695, but not APP751, mRNA transiently changes from a more homogeneous distribution at birth to a pronounced laminar pattern with higher mRNA levels in cortical layer III/IV detectable at the age of 4 days and persisting until postnatal day 10. The distinct age-related changes in cortical APP695 and APP751 mRNA levels reflect the functional alterations during early brain maturation and suggest that APP695 might play a role in establishing the mature connectional pattern between neurons, whereas APP751 could play a role in controlling cellular growth and synaptogenesis. Lesion of basal forebrain cholinergic system by the selective cholinergic immunotoxin 192IgG-saporin resulted in decreased levels of APP695 but not APP751 and APP770 transcripts by about 15-20% in some cortical (cingulate, frontal, parietal, piriform cortex), hippocampal regions (CA1, dentate gyrus), and basal forebrain nuclei (medial septum, vertical limb of diagonal band), detectable not earlier than 30 days after lesion and persisting until 90 days postlesion, suggesting that the nearly complete loss of cortical cholinergic input does not have any significant impact on the expression of APP mRNA isoforms in cholinoceptive cortical target regions.

Expression of a dominant negative FGF receptor inhibits axonal growth and FGF receptor phosphorylation stimulated by CAMs

The cell adhesion molecules (CAMs) NCAM, N-cadherin, and L1 are homophilic binding molecules that stimulate axonal growth. We have postulated that the above CAMs can stimulate this response by activating the fibroblast growth factor receptor (FGFR) in neurons. In the present study, we demonstrate that activation of NCAM and L1 can lead to phosphorylation of the FGFR. Both this and the neurite outgrowth response stimulated by all three of the above CAMs are lost when a kinase-deleted, dominant negative form of FGFR1 is expressed in PC12 cells. In addition, we have generated transgenic mice that express the dominant negative FGFR under control of the neuron-specific enolase (NSE) promoter. We show that cerebellar neurons isolated from these mice have also lost their ability to respond to NCAM, N-cadherin, and L1. A peptide inhibitor of phospholipase C gamma (PLCgamma) that inhibits neurite outgrowth stimulated by FGF also inhibited neurite outgrowth stimulated by the CAMs. Thus, we conclude that activation of the FGFR is both necessary and sufficient to account for the ability of the above CAMs to stimulate axonal growth, and that PLCgamma is a key downstream effector of this response.

Comparison of neurodegenerative pathology in transgenic mice overexpressing V717F beta-amyloid precursor protein and Alzheimer's disease

Overexpression of mutated human amyloid precursor protein (hAPP717V-->F) under control of platelet-derived growth factor promoter (PDAPP minigene) in transgenic (tg) mice results in neurodegenerative changes similar to Alzheimer's disease (AD). To clarify the pathology of these mice, we studied images derived from laser scanning confocal and electron microscopy and performed comparisons between PDAPP tg mice and AD. Similar to AD, neuritic plaques in PDAPP tg mouse contained a dense amyloid core surrounded by anti-hAPP- and antineurofilament-immunoreactive dystrophic neurites and astroglial cells. Neurons were found in close proximity to plaques in PDAPP tg mice and, to a lesser extent, in AD. In PDAPP tg mice, and occasionally in AD, neuronal processes contained fine intracellular amyloid fibrils in close proximity to the rough endoplasmic reticulum, coated vesicles, and electron-dense material. Extracellular amyloid fibrils (9-11 nm in diameter) were abundant in PDAPP tg and were strikingly similar to those observed in AD. Dystrophic neurites in plaques of PDAPP tg mouse and AD formed synapses and contained many dense multilaminar bodies and neurofilaments (10 nm). Apoptotic-like figures were present in the tg mice. No paired helical filaments have yet been observed in the heterozygote PDAPP tg mice. In summary, this study shows that PDAPP tg mice develop massive neuritic plaque formation and neuronal degeneration similar to AD. These findings show that overproduction of hAPP717V-->F in tg mice is sufficient to cause not only amyloid deposition, but also many of the complex subcellular degenerative changes associated with AD.

Beta-secretase processing of the beta-amyloid precursor protein in transgenic mice is efficient in neurons but inefficient in astrocytes

Alzheimer's disease is characterized by the extracellular deposition of beta-amyloid peptide (Abeta) in cerebral plaques and evidence is accumulating that amyloid is neurotoxic. Abeta is derived from the beta-amyloid precursor protein (APP). Proteolytic processing of APP by the enzyme, beta-secretase, produces the N terminus of Abeta, and releases a secreted ectodomain of APP (beta-s-APP). To develop animal models for measuring beta-secretase activity in specific brain cells in vivo, we have targeted the expression of the full-length human APP to either neurons or astrocytes in transgenic mice using the neuron- specific enolase (NSE) promoter or a modified glial fibrillary acidic protein (GFAP) gene, respectively. The APP cDNAs expressed were mutated (KM to NL at 670/671) to encode amino acid substitutions that enhance amyloidogenic processing in vitro. Western analyses revealed abundant production of beta-s-APP in the brains of NSE-APP mice and enzyme-linked immunosorbent assay analyses showed production of Abeta in fetal primary mixed brain cultures and brain homogenates from these transgenic animals. Because the NSE promoter drives expression primarily in neurons, this provides in vivo evidence that the beta-secretase cleavage necessary for generation of beta-s-APP and Abeta is efficiently performed in neurons. In contrast, only little beta-s-APP was detected in brain homogenates of GFAP-APP mice, indicating that astrocytes show very little beta-secretase activity in vivo. This provides strong in vivo evidence that the major source of Abeta in brain is from neurons and not from astrocytes.

Alterations in apolipoprotein E expression during aging and neurodegeneration

Apolipoprotein E (apoE) is a 34 kDa protein that plays an important role in cholesterol transport, uptake and redistribution. Within the nervous system, apoE might be involved in maintaining synaptic integrity after injury and during aging. ApoE might help maintain the integrity of the synaptodendritic complex by several different mechanisms. Among them, recent studies have suggested that apoE: (1) stabilizes the neuronal cytoskeleton; (2) plays an important role in transporting esterified cholesterol to neurons undergoing reinnervation where it is taken up by the low density lipoprotein receptor-related protein pathway and used as a precursor for the synthesis of new synaptic terminals; (3) regulates interactions between neurons and the extracellular matrix (e.g. laminin); and (4) regulates levels of intracellular calcium. The main objective of the manuscript is to review the current progress in understanding the functions of apoE in the nervous system and how malfunctioning of this molecule might result in neurodegenerative disorders such as Alzheimer's disease.

Conditional ablation of cerebellar astrocytes in postnatal transgenic mice

Astrocytes have been proposed to have multiple roles in the development and maintenance of the vertebrate CNS. To facilitate documentation of these roles, we designed a transgene to enable their ablation at selectable times. The transgene consists of the coding region for the herpes simplex virus-thymidine kinase (HSV-TK) under the control of the human glial fibrillary acidic protein gene promoter. The HSV-TK is innocuous but converts the antiherpetic agent ganciclovir (GCV) to a toxic product that interferes with DNA replication in proliferating cells. In a developmental study, transgenic mice were treated with GCV during the first postnatal week, with evaluation at P19. Treated mice displayed severe ataxia. Histological examination revealed disrupted astrocyte development, particularly in the cerebellum, with marked secondary effects on other cell types. Cerebellar defects included a loss in the numbers of astrocytes and an overall reduction in cerebellar size and disruption of the normally well defined cellular layers. Radial glia were disordered, Purkinje cells were ectopically distributed and displayed abnormal dendritic trees, and granule cells were markedly depleted. These effects were more severe in animals treated on postnatal day 1 versus treatment at day 5. A major factor causing granule cell death was excitotoxicity attributable to activation of NMDA receptors. These results suggest a critical role for astrocytes in cerebellar development.

Deficient glutamate transport is associated with neurodegeneration in Alzheimer's disease

The mechanisms of synapse damage in Alzheimer's disease (AD) are not fully understood. Deficient functioning of glutamate transporters might be involved in synaptic pathology and neurodegeneration by failing to clear excess glutamate at the synaptic cleft. In AD, glutamate transporter activity as assessed by D-[3H]aspartate binding is decreased; however, it is not clear to what extent it is associated with the neurodegenerative process and cognitive alterations. For this purpose, levels of D- and L-[3H]aspartate binding in midfrontal cortex were correlated with synaptophysin levels, brain spectrin degradation product levels, and clinical and neuropathological indicators of AD. Compared to control brains, AD brains displayed a 34% decrease in levels of D-[3H]aspartate binding, a 30% decrease in L-[3H]aspartate binding, and a 48% loss of synaptophysin immunoreactivity. Increased levels of brain spectrin degradation products correlated with a decrease in levels of D-[3H] and L-[3H]aspartate binding, and decreased levels of synaptophysin immunoreactivity. Levels of L-[3H]aspartate binding correlated with levels of synaptophysin immunoreactivity. These results suggest that decreased glutamate transporter activity in AD is associated with increased excitotoxicity and neurodegeneration, supporting the possibility that abnormal functioning of this system might be involved in the pathogenesis of synaptic damage in AD.

Enhanced amyloidogenic processing of the beta-amyloid precursor protein in gene-targeted mice bearing the Swedish familial Alzheimer's disease mutations and a "humanized" Abeta sequence

The processing of the beta-amyloid precursor protein (APP) in vivo has been characterized in a novel animal model that recapitulates, in part, the APP genotype of a familial form of Alzheimer's disease (AD). A gene-targeting strategy was used to introduce the Swedish familial AD mutations and convert mouse Abeta to the human sequence. The mutant APP is expressed at normal levels in brain, and cleavage at the mutant beta-secretase site is both accurate and enhanced. Furthermore, human Abeta production is significantly increased to levels 9-fold greater than those in normal human brain while nonamyloidogenic processing is depressed. The results on Abeta production extend similar findings obtained in cell culture to the brain of an animal and substantiate Abeta as a etiological factor in Swedish familial AD. These animals provide several distinguishing features over others created by conventional transgenic methodologies. The spatial and temporal expression patterns of human Abeta are expected to be faithfully reproduced because the gene encoding the mutant APP remains in its normal chromosomal context. Thus, the neuropathological consequences of human Abeta overproduction can be evaluated longitudinally in the absence of potential mitigating effects of APP overexpression or presence of the mouse Abeta peptide.

ONO-1603, a potential antidementia drug, shows neuroprotective effects and increases m3-muscarinic receptor mRNA levels in differentiating rat cerebellar granule neurons

We have reported that the antidementia drug tetrahydroaminoacridine (THA; 30 microM) is neuroprotective and neurotrophic and selectively increases m3-muscarinic acetylcholine receptor (mAChR) mRNA levels in differentiating cerebellar granule cells. Here, we examined whether novel prolyl endopeptidase inhibitor ONO-1603, a potential antidementia drug, induces similar effects in these cerebellar neurons. Supplement of ONO-1603 (0.03 microM) to cultures grown in 15 mM KCl-containing media was found to markedly promote neuronal survival and neurite outgrowth and enhance [3H]N-methylscopolamine binding to mAChRs. Moreover, ONO-1603 increased the level of m3-mAChR mRNA and stimulated mAChR-mediated phosphoinositide turnover. The common actions of ONO-1603 and THA suggest that these properties could be related to their putative antidementia activities and that this model system may be used to screen for drugs effective in the treatment for Alzheimer's disease.

APP-BP1, a novel protein that binds to the carboxyl-terminal region of the amyloid precursor protein

beta-Amyloid protein precursors (APPs, 695-770 amino acids) are the source of the 39-43 amino acid beta-amyloid (A beta) peptides that comprise diffuse and fibrillar deposits in the cerebral cortex and vasculature of Alzheimer's disease brains. A beta is thought to play a role in the pathogenesis of Alzheimer's disease, and, hence, considerable effort has been invested in defining the means by which A beta is generated from the APPs. Knowledge of the normal function of the APPs is sure to provide insights into the genesis and pathological persistence of A beta in Alzheimer's disease. APP is a cell surface protein with a large extracellular amino-terminal domain, a single transmembrane segment, and a short cytoplasmic tail. Its location and structural features characteristic of a receptor for signal transduction led us to search for potential effector proteins capable of binding and interacting with its cytoplasmic domain. Here, we report the cloning of a cDNA encoding one such protein. This ubiquitously expressed 59-kDa APP-binding protein, called APP-BP1, is 61% similar to a protein encoded by the Arabidopsis AXR1 gene, required for normal response to the hormone auxin, and is a relative of the ubiquitin activating enzyme E1.

A beta A4 amyloid precursor protein gene and Alzheimer's disease

Alzheimer's disease is a senile dementia caused by progressive neurodegeneration of the central nervous system. One of the most prominent pathological characteristics is beta A4 amyloid deposition in senile plaques in the brain parenchyma and in cerebral blood vessels. beta A4 amyloid is processed from a larger integral membrane protein, the beta A4 amyloid precursor protein. Different pathogenic mutations in this protein have been detected in a small number of Alzheimer's disease families. Here functional implications of these mutations on the processing of the precursor protein and the beta A4 amyloid deposition will be discussed with respect to the pathogenesis of Alzheimer's disease and related disorders.

Expression of APP in transgenic mice: a comparison of neuron-specific promoters

The beta-amyloid precursor protein (APP) carries mutations in codons 717 or 670/671, which cosegregate with familial forms of Alzheimer's disease (AD). As an initial step to study the related pathogenetic mechanisms in vivo we have generated transgenic mice expressing APP with these mutations. Several neuron-specific promoters were used to drive expression of human APP cDNAs. Only the Thy-1 promoter yielded transgene expression levels comparable to or above the endogenous mouse levels. Deletion of a 121 bp sequence from the 3' untranslated region of APP appeared to increase mRNA levels. Transgene mRNA was found throughout the brain with highest levels in hippocampus and cerebral cortex. Accordingly, human APP was detected in these regions by Western blotting. Protein levels paralleled mRNA levels reaching or exceeding the amount of endogenous APP. Variable reactivity of human APP in cell bodies was shown by immunocytochemistry. Although our initial histological examinations did not reveal any alterations characteristic of AD, further studied will be required.

Serotonin 5-HT2a and 5-HT2c receptors stimulate amyloid precursor protein ectodomain secretion

Alzheimer's disease amyloid consists of amyloid beta-peptides (Abeta) derived from the larger precursor amyloid precursor protein (APP). Non-amyloidogenic APP processing involves regulated cleavage within the Abeta domain followed by secretion of the ectodomain (APPs). APPs secretion can be stimulated by muscarinic acetylcholine receptors coupled to phospholipases and kinases. To determine whether other receptor classes can regulate APP processing, we examined the relation between serotonin receptors and APPs secretion. Serotonin increased APPs release 3-4-fold in 3T3 cells stably overexpressing 5-HT2aR or 5-HT2cR. The increase was dose-dependent and was blocked by serotoninergic antagonists. Phorbol esters also increased APPs secretion, but neither kinase inhibitors nor down-regulation of PKC blocked the serotonin-induced increase in APPs secretion. Thus PKC is not necessary to stimulate APPs secretion. Phospholipase A2 (PLA2) inhibitors blocked the 5-HT2aR-mediated increase in APPs secretion, suggesting a role of PLA2 in coupling 5-HT2aR to APP processing. In contrast, coupling of 5-HT2cR to APPs secretion involved both PKC and PLA2. Serotonin also stimulated the release of the APLP2 ectodomain, suggesting that additional members of the APP multigene family are processed via similar regulated pathways. Inasmuch as generation of APPs precludes the formation of amyloidogenic derivatives, serotonin receptors provide a novel pharmacological target to reduce these derivatives in Alzheimer's disease.

Neurotrophic and neuroprotective effects of hAPP in transgenic mice

To better understand the role the human amyloid precursor protein (hAPP) plays in Alzheimer's disease (AD), it is essential to define its primary function(s). Here we expressed different hAPPs in neurons of transgenic (tg) mice to characterize their effects on the intact central nervous system (CNS). Immunolabeled brain sections of tg and non-tg mice were compared quantitatively by microdensitometry and computer-aided analysis of laser scanning confocal digitized images. Compared with non-tg mice, tg mice overexpressing hAPPs showed an increase in the number of synaptophysin immunoreactive presynaptic terminals as well as in the expression of the growth-associated marker GAP-43. While non-tg controls and tg mice expressing hAPP751 at moderate levels displayed a normal pattern of reinnervation of the dentate gyrus following perforant pathway transection, tg mice expressing hAPP695 at severalfold higher levels showed an accentuation of the synaptic loss and no sprouting reaction. In addition, expression of hAPP751 at moderate levels effectively protected neurons against excitotoxic injury induced either acutely by systemic injection of kainic acid or chronically by transgene-driven glial production of the soluble HIV-1 protein gp120. Neuronal expression of hAPP695 at higher levels provided less excitoprotection. Our findings are consistent with the postulate that APP plays a role in the formation/maintenance of synapses and that processes which affect this function could contribute to the synaptic pathology seen in AD. Our study also revealed that hAPPs can exert important excitoprotective functions in vivo and that the efficiency of this protection may depend on the hAPP isoform expressed as well as on the level of neuronal hAPP expression. Neuronal overexpression of hAPP beyond a certain level may have detrimental effects on the CNS, particularly in the context of secondary neural injuries.

Activation of K+ channels and suppression of neuronal activity by secreted beta-amyloid-precursor protein

The Alzheimer's beta-amyloid precursor protein (beta-APP) is widely expressed in neural cells, and in neurons secreted forms of beta-APP (sAPPs) are released from membrane-spanning holo-beta APP in an activity-dependent manner. Secreted APPs can modulate neurite outgrowth, synaptogenesis, synaptic plasticity and cell survival; a signal transduction mechanism of sAPPs may involve modulation of intracellular calcium levels ([Ca2+]i). Here we use whole-cell perforated patch and single-channel patch-clamp analysis of hippocampal neurons to demonstrate that sAPPs suppress action potentials and hyperpolarize neurons by activating high-conductance, charybdotoxin-sensitive K+ channels. Activation of K+ channels by sAPPs was mimicked by a cyclic GMP analogue and sodium nitroprusside and blocked by an antagonist of cGMP-dependent kinase and a phosphatase inhibitor, suggesting that the effect is mediated by cGMP and protein dephosphorylation. Calcium imaging studies indicate that activation of K+ channels mediates the ability of sAPPs to decrease [Ca2+]i. Modulation of neuronal excitability may be a major mechanism by which beta-APP regulates developmental and synaptic plasticity in the nervous system.