Co-expression of truncated and full-length tau induces severe neurotoxicity

Abundant tau inclusions are a defining hallmark of several human neurodegenerative diseases, including Alzheimer's disease. Protein fragmentation is a widely observed event in neurodegenerative proteinopathies. The relevance of tau fragmentation for the neurodegenerative process in tauopathies has yet remained unclear. Here we found that co-expression of truncated and full-length human tau in mice provoked the formation of soluble high-molecular-weight tau, the failure of axonal transport, clumping of mitochondria, disruption of the Golgi apparatus and missorting of synaptic proteins. This was associated with extensive nerve cell dysfunction and severe paralysis by the age of 3 weeks. When the expression of truncated tau was halted, most mice recovered behaviorally and functionally. In contrast, co-expression of full-length tau isoforms did not result in paralysis. Truncated tau thus induces extensive but reversible neurotoxicity in the presence of full-length tau through the formation of nonfilamentous high-molecular-weight tau aggregates, in the absence of tau filaments. Targeting tau fragmentation may provide a novel approach for the treatment of human tauopathies.

Exposure to 56Fe-particle radiation accelerates electrophysiological alterations in the hippocampus of APP23 transgenic mice

Abstract An unavoidable complication of space travel is exposure to high-charge, high-energy (HZE) particles. In animal studies, exposure of the CNS to HZE-particle radiation leads to neurological alterations similar to those seen in aging or Alzheimer's disease. In this study we examined whether HZE-particle radiation accelerated the age-related neuronal dysfunction that was previously described in transgenic mice overexpressing human amyloid precursor protein (APP). These APP23 transgenic mice exhibit age-related behavioral abnormalities and deficits in synaptic transmission. We exposed 7-week-old APP23 transgenic males to brain-only (56)Fe-particle radiation (600 MeV/nucleon; 1, 2, 4 Gy) and recorded synaptic transmission in hippocampal slices at 2, 6, 9, 14 and 18-24 months. We stimulated Schaeffer collaterals and recorded field excitatory postsynaptic potentials (fEPSP) and population spikes (PS) in CA1 neurons. Radiation accelerated the onset of age-related fEPSP decrements recorded at the PS threshold from 14 months of age to 9 months and reduced synaptic efficacy. At 9 months, radiation also reduced PS amplitudes. At 6 months, we observed a temporary deficit in paired-pulse inhibition of the PS at 2 Gy. Radiation did not significantly affect survival of APP23 transgenic mice. We conclude that irradiation of the brain with HZE particles accelerates Alzheimer's disease-related neurological deficits.

Aged APP23 mice show a delay in switching to the use of a strategy in the Barnes maze

Spatial learning and memory deficits in the APP23 transgenic mice have mainly been studied using the Morris water maze (MWM). However learning in the MWM relies on swimming abilities and may be confounded by the stressful nature of this test. We have therefore assessed spatial learning and memory in 12-month-old APP23 using a dry-land maze test developed by Barnes. Mice were given daily learning trials for a total of 41 successive days. After a 12-day interval the mice were re-tested for 4 additional days in order to examine the spatial memory retention. Immediately following this phase, reversal learning was examined for 13 additional days by moving the escape tunnel to the opposite position. During the initial learning phase, APP23 mice showed a significantly longer latency to find the escape tunnel as well as an increased number of errors compared to non-transgenic littermates. These deficits appeared to be due to a delay in switching from a "no strategy" to a spatial strategy. Indeed, this same delay in the use of spatial strategy was observed in the reversal phase of the study. Our results suggest that impairments in APP23 mice in learning and memory maze tests may be due to a specific deficit in the use of spatial strategy.

Neurobehavioral characterization of APP23 transgenic mice with the SHIRPA primary screen

The SHIRPA primary screen comprises 40 measures covering various reflexes and basic sensorimotor functions. This multi-test battery was used to compare non-transgenic controls with APP23 transgenic mice, expressing the 751 isoform of human beta-amyloid precursor protein and characterized by amyloid deposits in parenchyma and vessel walls. The APP23 mice were distinguishable from controls by pathological limb reflexes, myoclonic jumping, seizure activity, and tail malformation. In addition, this mouse model of Alzheimer's disease was also marked by a crooked swimming trajectory. APP23 mice were also of lighter weight and were less inclined to stay immobile during a transfer arousal test. Despite the neurologic signs, APP23 transgenic mice were not deficient in stationary beam, coat-hanger, and rotorod tests, indicating intact motor coordination abilities.

Spatial learning and exploration of environmental stimuli in 24-month-old female APP23 transgenic mice with the Swedish mutation

Transgenic mice overexpressing the betaAPP gene with the Swedish mutation under the control of the murine thy1 promoter show Alzheimer-like characteristics such as Abeta plaques in cerebral cortex and vessel walls. By comparison to age-matched non-transgenic controls, 2-year-old female APP23 transgenic mice crossed more segments in the open-field and had a higher number of fast ambulatory and stereotyped movements in a photocell activity chamber. In addition, APP23 mice entered more often and spent more time in the open arms of the elevated plus-maze. The acquisition of place learning in the Morris water maze was impaired in APP23 transgenic mice, but not in probe and visible platform subtasks. These results indicate that hyperactivity and impaired learning abilities characterize this mouse model of Alzheimer's disease and cerebral angiopathy.

The LPS receptor (CD14) links innate immunity with Alzheimer's disease

To rapidly respond to invading microorganisms, humans call on their innate immune system. This occurs by microbe-detecting receptors, such as CD14, that activate immune cells to eliminate the pathogens. Here, we link the lipopolysaccharide receptor CD14 with Alzheimer's disease, a severe neurodegenerative disease resulting in dementia. We demonstrate that this key innate immunity receptor interacts with fibrils of Alzheimer amyloid peptide. Neutralization with antibodies against CD14 and genetic deficiency for this receptor significantly reduced amyloid peptide induced microglial activation and microglial toxicity. The observation of strongly enhanced microglial expression of the LPS receptor in brains of animal models of Alzheimer's disease indicates a clinical relevance of these findings. These data suggest that CD14 may significantly contribute to the overall neuroinflammatory response to amyloid peptide, highlighting the possibility that the enormous progress currently being made in the field of innate immunity could be extended to research on Alzheimer's disease.

Regional brain cytochrome oxidase activity in beta-amyloid precursor protein transgenic mice with the Swedish mutation

Cytochrome oxidase activity was examined in a transgenic mouse model of Alzheimer's disease with overexpression of the 751 amino acid isoform of beta-amyloid precursor protein with the Swedish mutation under control of the murine thy-1 promoter. The neuritic plaques, abundantly localized in the hippocampus and anterior neocortical areas, showed a core devoid of enzymatic activity surrounded by higher cytochrome oxidase activity at the sites of the dystrophic neurites and activated glial cells. Quantitative measures, taken only in the healthy-appearing regional areas without neuritic plaques, were higher in numerous limbic and non-limbic regions of transgenic mice in comparison with controls. Enzymatic activity was higher in the dentate gyrus and CA2-CA3 region of the hippocampus, the anterior cingulate and primary visual cortex, two olfactory structures, the ventral part of the neostriatum, the parafascicularis nucleus of the thalamus, and the subthalamic nucleus. Brainstem regions anatomically related with altered forebrain regions were more heavily labeled as well, including the substantia nigra, the periaqueductal gray, the superior colliculus, the medial raphe, the locus coeruleus and the adjacent parabrachial nucleus, as well as the pontine nuclei, red nucleus, and trigeminal motor nucleus. Functional brain organization is discussed in the context of Alzheimer's disease. Although hypometabolism is generally observed in this pathology, the increased cytochrome oxidase activity obtained in these transgenic mice can be the result of a functional compensation on the surviving neurons, or of an early mitochondrial alteration related to increased oxidative damage.

Progressive age-related impairment of cognitive behavior in APP23 transgenic mice

Transgenic APP23 mice expressing human APP(751) with the K670N/M671L mutation, were compared at ages 3, 18 or 25 months to non-transgenic littermates in passive avoidance and in a small and large Morris maze. The task in the smaller pool habituated their flight response to the platform. Impairments in passive avoidance and small pool performance in APP23 mice were clearly age-related. In the larger Morris maze APP23 mice at all ages were impaired in latency and distance swum before finding the platform. Identical performance of 18-month APP23 and controls in a visible platform condition indicates that the Morris maze performance deficit was not due to sensory, motor or motivational alterations. At age 3 months both groups initially unexpectedly avoided the visible platform, suggesting that in young mice neophobia may contribute significantly to performance in cognitive tests. In conclusion, APP23 mice exhibit both early behavioral impairment in the large Morris maze as well as impairments in passive avoidance and small pool performance that are marked only in old age.

Impaired hypoxic tolerance and altered protein binding of NADH in presymptomatic APP23 transgenic mice

It is being discussed whether impairment of energy metabolism is a final common pathway of neurodegeneration or initiates the neurodegenerative cascade. The goal was to investigate hypoxic tolerance and oxidative energy metabolism in 4-month-old, presymptomatic B6-Tg(ThylAPP)23Sdz (APP23) mice, a transgenic mouse model of Alzheimer's disease. Posthypoxic recovery of the population spike amplitude in hippocampal region CA1 upon stimulation of Schaffer collaterals in region CA3 (15 min hypoxia, 45 min recovery) was 43+/-46% (mean+/-S.D.) vs. 19+/-35% (P<0.05) in slices from wild-type and transgenic animals, respectively. Fluorescence lifetime sensitive spectroscopy of NADH in the CA1 pyramidal cell layer (gate set for detection of protein-bound NADH) showed a wavelength maximum at 455.3+/-1.6 nm (mean+/-S.D.) in controls and 453.5+/-2.4 nm (P<0.05) in mutants. We conclude that hypoxic tolerance is impaired in presymptomatic APP23 mice and occurs prior to extracellular deposition of amyloid plaques. Impaired energy metabolism may thus partake in initiating the neurodegenerative cascade in a transgenic model of Alzheimer's disease. The blue shift of the spectrum of NADH in mutant mice indicates an altered protein microenvironment of energy metabolism under control conditions.

Spatial learning, exploration, anxiety, and motor coordination in female APP23 transgenic mice with the Swedish mutation

Transgenic mice overexpressing the betaAPP gene with the Swedish mutation under the control of the murine thy-1 promoter show Alzheimer-like characteristics including the accumulation of Abeta protein in the cerebral cortex. Female 16-month-old APP23 transgenic mice were compared to age-matched non-transgenic mice in behavioral tests measuring spatial learning, exploration of environmental stimuli, anxiety, and motor coordination. APP23 transgenic mice had fewer fast ambulatory movements, either fast or slow stereotypy movements, and slow rears in a photocell activity chamber. The acquisition of spatial learning in the Morris water maze was impaired in APP23 transgenic mice, but not during the probe test or while swimming towards a visible platform. Neither were there intergroup differences in tests of anxiety or motor coordination. These results indicate that a learning deficit and hypoactivity, concordant with the early stages of Alzheimer's disease, characterize this mouse model with Abeta accumulation.

Expression of endothelial and inducible NOS-isoforms is increased in Alzheimer's disease, in APP23 transgenic mice and after experimental brain lesion in rat: evidence for an induction by amyloid pathology

The nitric oxide-synthesizing enzyme nitric oxide synthase (NOS) is present in the mammalian brain in three different isoforms, two constitutive enzymes (i.e., neuronal, nNOS, and endothelial eNOS) and one inducible enzyme (iNOS). All three isoforms are aberrantly expressed in Alzheimer's disease giving rise to elevated levels of nitric oxide apparently involved in the pathogenesis of this disease by various different mechanisms including oxidative stress and activation of intracellular signalling mechanisms. It still is a matter of debate, however, whether the abnormal expression of NOS isoforms has some primary importance in the pathogenetic chain and might thus be a potential therapeutic target or only reflects a secondary effect that occurs at more advanced stages of the disease process. To tackle this question, we analysed the expression of both eNOS and iNOS in patients with sporadic AD, in transgenic mice expressing human amyloid precursor protein (APP) with the Swedish double mutation under control of the Thy1 promotor (APP23 mice), and after electrolytic cortical lesion in rat, an experimental paradigm associated with elevated expression of APP. In all three conditions, an astrocytosis was induced accompanied by a strong increase of both iNOS and eNOS. Both NOS isoforms were frequently though not always colocalized. Thus, based on the expression pattern of NOS isoforms three types of astrocytes, expressing only one of the two isoforms or both together could be distinguished. In both AD and transgenic mice eNOS-expressing astrocytes exceeded iNOS-expressing astrocytes in number. Astrocytes with elevated levels of iNOS or eNOS were constantly seen in direct association with Abeta-deposits in AD and transgenic mice and were found in the vicinity of the lesion site in the rat cortex. The results of the present study show that expression of both iNOS and eNOS is increased in activated astrocytes under experimental conditions associated with elevated expression of APP (electrolytic brain lesion) or Abeta-deposition (APP23 transgenic mice). Therefore, it is suggested that altered expression of these NOS isoforms being part of AD pathology is secondary to the amyloid pathology and might not be primarily involved in the pathogenetic chain though it might contribute to the maintenance, self-perpetuation and progression of the neurodegenerative process.

The evolution of A beta peptide burden in the APP23 transgenic mice: implications for A beta deposition in Alzheimer disease

BACKGROUND

High levels of A beta in the cerebral cortex distinguish demented Alzheimer's disease (AD) from nondemented elderly individuals, suggesting that decreased amyloid-beta (A beta) peptide clearance from the brain is a key precipitating factor in AD.

MATERIALS AND METHODS

The levels of A beta in brain and plasma as well as apolipoprotein E (ApoE) in brain were investigated by enzyme-linked immunosorbent assay (ELISA) and Western blotting at various times during the life span of the APP23 transgenic (Tg) and control mice. Histochemistry and immunocytochemistry were used to assess the morphologic characteristics of the brain parenchymal and cerebrovascular amyloid deposits and the intracellular amyloid precursor protein (APP) deposits in the APP23 Tg mice.

RESULTS

No significant differences were found in the plasma levels of A beta between the APP23 Tg and control mice from 2-20 months of age. In contrast, soluble A beta levels in the brain were continually elevated, increasing 4-fold at 2 months and 33-fold in the APP23 Tg mice at 20 months of age when compared to the control mice. Soluble A beta42 was about 60% higher than A beta40. In the APP23 Tg mice, insoluble A beta40 remained at basal levels in the brain until 9 months and then rose to 680 microg/g cortex by 20 months. Insoluble A beta40 was negligible in non-Tg mice at all ages. Insoluble A beta42 in APP23 Tg mice rose to 60 microg/g cortex at 20 months, representing 24 times the control A beta42 levels. Elevated levels of ApoE in the brain were observed in the APP23 Tg mice at 2 months of age, becoming substantially higher by 20 months. ApoE colocalized with A beta in the plaques. Beta-amyloid precursor protein (betaAPP) deposits were detected within the neuronal cytoplasm from 4 months of age onward. Amyloid angiopathy in the APP23 Tg mice increased markedly with age, being by far more severe than in the Tg2576 mice.

CONCLUSIONS

We suggest that the APP23 Tg mouse may develop an earlier blockage in A beta clearance than the Tg2576 mice, resulting in a more severe accumulation of A beta in the perivascular drainage pathways and in the brain. Both Tg mice reflect decreased A beta elimination and as models for the amyloid cascade they are useful to study AD pathophysiology and therapy.

3D-Reconstruction of microglia and amyloid in APP23 transgenic mice: no evidence of intracellular amyloid

Microglia cells are closely associated with compact amyloid plaques in Alzheimer's disease (AD) brains. Although activated microglia seem to play a central role in the pathogenesis of AD, mechanisms of microglial activation by beta-amyloid as well as the nature of interaction between amyloid and microglia remain poorly understood. We previously reported a close morphological association between activated microglia and congophilic amyloid plaques in the brains of APP23 transgenic mice at both the light and electron microscopic levels [25]. In the present study, we have further examined the structural relationship between microglia and amyloid deposits by using postembedding immunogold labeling, serial ultrathin sectioning, and 3-dimensional reconstruction. Although bundles of immunogold-labeled amyloid fibrils were completely engulfed by microglial cytoplasm on single sections, serial ultrathin sectioning and three-dimensional reconstruction revealed that these amyloid fibrils are connected to extracellular amyloid deposits. These data demonstrate that extracellular amyloid fibrils form a myriad of finger-like channels with the widely branched microglial cytoplasm. We conclude that in APP23 mice a role of microglia in amyloid phagocytosis and intracellular production of amyloid is unlikely.

Comparative analysis of amyloid-beta chemical structure and amyloid plaque morphology of transgenic mouse and Alzheimer's disease brains

We have undertaken an integrated chemical and morphological comparison of the amyloid-beta (Abeta) molecules and the amyloid plaques present in the brains of APP23 transgenic (tg) mice and human Alzheimer's disease (AD) patients. Despite an apparent overall structural resemblance to AD pathology, our detailed chemical analyses revealed that although the amyloid plaques characteristic of AD contain cores that are highly resistant to chemical and physical disruption, the tg mice produced amyloid cores that were completely soluble in buffers containing SDS. Abeta chemical alterations account for the extreme stability of AD plaque core amyloid. The corresponding lack of post-translational modifications such as N-terminal degradation, isomerization, racemization, pyroglutamyl formation, oxidation, and covalently linked dimers in tg mouse Abeta provides an explanation for the differences in solubility between human AD and the APP23 tg mouse plaques. We hypothesize either that insufficient time is available for Abeta structural modifications or that the complex species-specific environment of the human disease is not precisely replicated in the tg mice. The appraisal of therapeutic agents or protocols in these animal models must be judged in the context of the lack of complete equivalence between the transgenic mouse plaques and the human AD lesions.

Pathogenic mechanisms of Alzheimer's disease analyzed in the APP23 transgenic mouse model

APP23 transgenic mice overexpress human APP with the Swedish double mutation. The mice start to develop amyloid plaque pathology at about six months of age, followed somewhat later by vascular amyloid deposits. Plaques are mostly of the compact type and increase exponentially during aging. Female mice show a slightly more rapid A beta plaque deposition than do male animals. Associated with the amyloid are inflammatory reactions, neuritic and synaptic degeneration as well as tau hyperphosphorylation. Older mice have a reduced cholinergic fiber length and a reduced neuron number in the hippocampal CA1 region. Crossbreeding with transgenic mice expressing human presenilin 1 carrying Alzheimer's disease-linked mutations lead to an enhancement of the pathology. The APP23 line is a suitable model to analyze the contribution of APP, A beta, and amyloid to the pathogenesis of Alzheimer's disease.

Spontaneous hemorrhagic stroke in a mouse model of cerebral amyloid angiopathy

A high risk factor for spontaneous and often fatal lobar hemorrhage is cerebral amyloid angiopathy (CAA). We now report that CAA in an amyloid precursor protein transgenic mouse model (APP23 mice) leads to a loss of vascular smooth muscle cells, aneurysmal vasodilatation, and in rare cases, vessel obliteration and severe vasculitis. This weakening of the vessel wall is followed by rupture and bleedings that range from multiple, recurrent microhemorrhages to large hematomas. Our results demonstrate that, in APP transgenic mice, the extracellular deposition of neuron-derived beta-amyloid in the vessel wall is the cause of vessel wall disruption, which eventually leads to parenchymal hemorrhage. This first mouse model of CAA-associated hemorrhagic stroke will now allow development of diagnostic and therapeutic strategies.

Abeta-induced inflammatory processes in microglia cells of APP23 transgenic mice

A microglial response is part of the inflammatory processes in Alzheimer's disease (AD). We have used APP23 transgenic mice overexpressing human amyloid precursor protein with the Swedish mutation to characterize this microglia response to amyloid deposits in aged mice. Analyses with MAC-1 and F4/80 antibodies as well as in vivo labeling with bromodeoxyuridine demonstrate that microglia in the plaque vicinity are in an activated state and that proliferation contributes to their accumulation at the plaque periphery. The amyloid-induced microglia activation may be mediated by scavenger receptor A, which is generally elevated, whereas the increased immunostaining of the receptor for advanced glycation end products is more restricted. Although components of the phagocytic machinery such as macrosialin and Fc receptors are increased in activated microglia, efficient clearance of amyloid is missing seemingly because of the lack of amyloid-bound autoantibodies. Similarly, although up-regulation of major histocompatibility complex class II (IA) points toward an intact antigen-presenting function of microglia, lack of T and B lymphocytes does not indicate a cell-mediated immune response in the brains of APP23 mice. The similar characteristics of microglia in the APP23 mice and in AD render the mouse model suitable to study the role of inflammatory processes during AD pathogenesis.

Transgenic mouse models of Alzheimer's disease

Alzheimer's disease (AD) pathology is characterized by A beta peptide-containing plaques, neurofibrillary tangles consisting of hyperphosphorylated tau, extensive neuritic degeneration, and distinct neuron loss. We generated several transgenic mouse lines expressing the human amyloid precursor protein (APP751) containing the AD-linked KM670/671NL double mutation (Swedish mutation) under the control of a neuron-specific Thy-1 promoter fragment. In the best APP-expressing line (APP23), compact A beta deposits can be detected at 6 months of age. These plaques dramatically increase with age, are mostly Congo Red positive, and accumulate typical plaque-associated proteins such as heparansulfate proteoglycan and apolipoprotein E. Activated astrocytes and microglia indicative of inflammatory processes reminiscent of AD accumulate around the deposits. Furthermore, plaques are surrounded by enlarged dystrophic neurites as visualized by neurofilament or Holmes-Luxol staining. Strong staining for acetylcholinesterase activity is found throughout the plaques and is accompanied by local distortion of the cholinergic fiber network. All congophilic plaques contain hyperphosphorylated tau reminiscent of early tau pathology. Modern stereologic methods demonstrate a significant loss of neurons in the hippocampal CA1 region, correlating with an increasing A beta plaque load. Interestingly, APP23 mice develop cerebral amyloid angiopathy in addition to amyloid plaques even though the APP transgene is only expressed in neurons. Crossbreeding of APP23 mice with transgenic mice carrying AD-linked presenilin mutations but not wild-type presenilin resulted in enhanced formation of pathology. In conclusion, our APP transgenic mice present many pathologic features, similar to those observed in AD and therefore offer excellent tools for studying the contribution of A beta to AD pathogenesis.

Amyloid beta interacts with the amyloid precursor protein: a potential toxic mechanism in Alzheimer's disease

Amyloid beta protein (Abeta) deposition in the brain is a hallmark of Alzheimer's disease (AD). The fibrillar form of Abeta is neurotoxic, although the mechanism of its toxicity is unknown. We showed that conversion of Abeta to the fibrillar form markedly increased binding to specific neuronal membrane proteins, including amyloid precursor protein (APP). Nanomolar concentrations of fibrillar Abeta bound cell-surface holo-APP in cortical neurons. Reduced vulnerability of cultured APP-null neurons to Abeta neurotoxicity suggested that Abeta neurotoxicity involves APP. Thus Abeta toxicity may be mediated by the interaction of fibrillar Abeta with neuronal membrane proteins, notably APP. An Abeta-APP interaction reminiscent of the pathogenic mechanism of prions may thus contribute to neuronal degeneration in AD.

Transgenic approaches to model Alzheimer's disease

Two transgenic mouse lines were generated which express human APP751 containing familial Alzheimer's disease (AD) mutations in brain neurons. These mice develop pathological features reminiscent of AD. The degree of pathology depends on both expression levels and specific mutations. In mice with more advanced pathology (APP 23), typical plaques appear at six 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. A quantitative analysis of degenerative changes by state-of-the-art unbiased stereological methods revealed a significant reduction in neuronal cell bodies of the CA1 field of the hippocampus when compared to controls. This reduction is directly related to plaque load. When subjected to analysis in the Morris water maze, 18 month old APP 23 mice show a significant increase in platform finding latency throughout the entire trial when compared to non-transgenic littermates.

Using β-Secretase Inhibitors to Distiguish the Generation of the Aβ Peptides Terminating at Val-40 and Ala-42

A large body of evidence suggests a causative role of β-amyloid (Aβ) in the pathogenesis of Alzheimer's disease (reviewed in refs. 1 and 2). Aβ is neurotoxic and toxicity requires the formation of amyloid fibrils similar to those found in senile plaques (3). Autosomal dominant mutations linked to Alzheimer's disease were identified in three different genes (4 ,5). All mutations apparently alter amyloid precursor protein (APP) metabolism to increase the generation of Aβ peptides terminating at amino acid Ala-42. Due to the tendency of the longer Aβ peptides to more readily form fibrils (7), these may accelerate Aβ deposition, which ultimately leads to more aggressive, early onset forms of Alzheimer's disease (8). With the transgenic expression of APP in mice this was explored further (9). Whereas a twofold overexpression of APP did not lead to Aβ deposition, the same quantitative expression of APP with a mutation at codon 717 known to increase the formation of Aβ42 led to the appearance of Aβ deposits at the age of 18 mo. These data suggest that the Aβ load in the brain as well as the amyloidogenic properties of the Aβ isoforms directly regulate deposition and senile plaque formation.

Neuronal overexpression of mutant amyloid precursor protein results in prominent deposition of cerebrovascular amyloid

Transgenic mice that overexpress mutant human amyloid precursor protein (APP) exhibit one hallmark of Alzheimer's disease pathology, namely the extracellular deposition of amyloid plaques. Here, we describe significant deposition of amyloid beta (Abeta) in the cerebral vasculature [cerebral amyloid angiopathy (CAA)] in aging APP23 mice that had striking similarities to that observed in human aging and Alzheimer's disease. Amyloid deposition occurred preferentially in arterioles and capillaries and within individual vessels showed a wide heterogeneity (ranging from a thin ring of amyloid in the vessel wall to large plaque-like extrusions into the neuropil). CAA was associated with local neuron loss, synaptic abnormalities, microglial activation, and microhemorrhage. Although several factors may contribute to CAA in humans, the neuronal origin of transgenic APP, high levels of Abeta in cerebrospinal fluid, and regional localization of CAA in APP23 mice suggest transport and drainage pathways rather than local production or blood uptake of Abeta as a primary mechanism underlying cerebrovascular amyloid formation. APP23 mice on an App-null background developed a similar degree of both plaques and CAA, providing further evidence that a neuronal source of APP/Abeta is sufficient to induce cerebrovascular amyloid and associated neurodegeneration.

Cerebral amyloid induces aberrant axonal sprouting and ectopic terminal formation in amyloid precursor protein transgenic mice

A characteristic feature of Alzheimer's disease (AD) is the formation of amyloid plaques in the brain. Although this hallmark pathology has been well described, the biological effects of plaques are poorly understood. To study the effect of amyloid plaques on axons and neuronal connectivity, we have examined the axonal projections from the entorhinal cortex in aged amyloid precursor protein (APP) transgenic mice that exhibit cerebral amyloid deposition in plaques and vessels (APP23 mice). Here we report that entorhinal axons form dystrophic boutons around amyloid plaques in the entorhinal termination zone of the hippocampus. More importantly, entorhinal boutons were found associated with amyloid in ectopic locations within the hippocampus, the thalamus, white matter tracts, as well as surrounding vascular amyloid. Many of these ectopic entorhinal boutons were immunopositive for the growth-associated protein GAP-43 and showed light and electron microscopic characteristics of axonal terminals. Our findings suggest that (1) cerebral amyloid deposition has neurotropic effects and is the main cause of aberrant sprouting in AD brain; (2) the magnitude and significance of sprouting in AD have been underestimated; and (3) cerebral amyloid leads to the disruption of neuronal connectivity which, in turn, may significantly contribute to AD dementia.

Association of microglia with amyloid plaques in brains of APP23 transgenic mice

Microglia are a key component of the inflammatory response in the brain and are associated with senile plaques in Alzheimer's disease (AD). Although there is evidence that microglial activation is important for the pathogenesis of AD, the role of microglia in cerebral amyloidosis remains obscure. The present study was undertaken to investigate the relationship between beta-amyloid deposition and microglia activation in APP23 transgenic mice which express human mutated amyloid-beta precursor protein (betaPP) under the control of a neuron-specific promoter element. Light microscopic analysis revealed that the majority of the amyloid plaques in neocortex and hippocampus of 14- to 18- month-old APP23 mice are congophilic and associated with clusters of hypertrophic microglia with intensely stained Mac-1- and phosphotyrosine-positive processes. No association of such activated microglia was observed with diffuse plaques. In young APP23 mice, early amyloid deposits were already of dense core nature and were associated with a strong microglial response. Ultrastructurally, bundles of amyloid fibrils, sometimes surrounded by an incomplete membrane, were observed within the microglial cytoplasm. However, microglia with the typical characteristics of phagocytosis were associated more frequently with dystrophic neurites than with amyloid fibrils. Although the present observations cannot unequivocally determine whether microglia are causal, contributory, or consequential to cerebral amyloidosis, our results suggest that microglia are involved in cerebral amyloidosis either by participating in the processing of neuron-derived betaPP into amyloid fibrils and/or by ingesting amyloid fibrils via an uncommon phagocytotic mechanism. In any case, our observations demonstrate that neuron-derived betaPP is sufficient to induce not only amyloid plaque formation but also amyloid-associated microglial activation similar to that reported in AD. Moreover, our results are consistent with the idea that microglia activation may be important for the amyloid-associated neuron loss previously reported in these mice.

Destabilization of beta-catenin by mutations in presenilin-1 potentiates neuronal apoptosis

Mutations of the presenilin-1 gene are a major cause of familial early-onset Alzheimer's disease. Presenilin-1 can associate with members of the catenin family of signalling proteins, but the significance of this association is unknown. Here we show that presenilin-1 forms a complex with beta-catenin in vivo that increases beta-catenin stability. Pathogenic mutations in the presenilin-1 gene reduce the ability of presenilin-1 to stabilize beta-catenin, and lead to increased degradation of beta-catenin in the brains of transgenic mice. Moreover, beta-catenin levels are markedly reduced in the brains of Alzheimer's disease patients with presenilin-1 mutations. Loss of beta-catenin signalling increases neuronal vulnerability to apoptosis induced by amyloid-beta protein. Thus, mutations in presenilin-1 may increase neuronal apoptosis by altering the stability of beta-catenin, predisposing individuals to early-onset Alzheimer's disease.

Amyloid beta peptide is not a candidate for the neurotrophic activities released from chromaffin cells

We have previously shown that chromaffin cells, the neuron-like cells of the adrenal medulla, release proteins, which promote in vitro survival of a large number of peripheral and central nervous system neurons (cf. Lachmund, A., Gehrke, D., Krieglstein, K. and Unsicker, K, Trophic factors from chromaffin granules promote survival of peripheral and central nervous system neurons. Neuroscience, 1994, 62, 361-370). In a search for the active molecules we are testing compounds that are known to be synthesized and released by chromaffin cells. Amyloid precursor protein (beta APP) is one of these factors (Bieger, S., Klafki, H.-W. and Unsicker, K., Synthesis and release of the beta-amyloid precursor protein by bovine chromaffin cells. Neurosci. Lett., 1993, 162, 173-175). In the present study we have investigated the possibility that amyloid beta peptide (A beta P) generated from beta APP may have survival supporting effects for neurons from embryonic chick ciliary (CG) and dorsal root ganglia (DRG). Embryonic rat hippocampal neurons, for which promotion of short-term survival by A beta P has been reported (Whitson, J. S., Selkoe, D. J. and Cotman, C. W., Amyloid beta protein enhances the survival of hippocampal neurons in vitro. Science, 1989, 243, 1488-1490), were employed as a reference. A beta P fragment 1-40, administered over a wide range of concentrations (1.5-100 micrograms/ml) did not promote the survival of CG and DRG neurons isolated from embryonic day (E) 8 chick embryos. The peptide also failed to toxically suppress survival of these neuron populations in the presence of survival promoting factors. In confirmation of previous reports, the 1-40 peptide, in comparison to the reverse 40-1 peptide, significantly enhanced survival of hippocampal neurons. These results suggest that A beta P is not a trophic factor for the peripheral neuron populations tested and most likely, not a neurotrophic component among the neurotrophic factors released by chromaffin cells.

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.

[Genetic predisposition to Alzheimer's disease]

INTRODUCTION

Alzheimer's disease can present either as an early-onset or senile dementia. Its ethiology is heterogeneous but with common clinical and pathological features.

DEVELOPMENT

Alzheimer's syndrome can occur with familial clustering. Some families with pre-senile dementia have shown three different genes which are associated with the pathological features. These genes carry on several mutations which have an autosomal dominant transmission: each mutation seems to be able to cause the pathological changes. The senile dementias are more common but dominant transmission has not been shown. However, risk genetic factors can play a rol. The epsilon 4 allel for apolipoprotein E has been clearly identified.

CONCLUSIONS

It would be possible to use genetic tests to predict the appearance of presenile dementias but these tests are not available for the more common senile types of Alzheimer's dementia.

Neuronal localization of presenilin-1 and association with amyloid plaques and neurofibrillary tangles in Alzheimer's disease

Mutations in the presenilin-1 (PS1) gene is a cause of early- onset familial Alzheimer's disease (AD). Endogenous PS1 is associated with the endoplasmic reticulum in the cell body of undifferentiated SH-SY5Y neuroblastoma cells. At early stages of neuronal differentiation in rat hippocampal culture, PS1 appears in all neuritic processes and in growth cones. In mature differentiated neurons, PS1 is concentrated in the somatodendritic compartment but is also present at lower levels in axons. A similar localization of PS1 is observed in vivo in neurons of the adult human cerebral cortex. In sporadic AD, PS1 appears in the dystrophic neurites of mature amyloid plaques and co-localizes with a subset of intraneuronal neurofibrillary tangles (NFTs). About 30% of hippocampal NFTs are labeled with a highly specific antibody to the PS1 C-terminal loop domain but not with an antibody to the PS1 N terminus. This observation is consistent with a potential association of the PS1 C-terminal fragment with NFTs, because PS1 is constitutively cleaved to N- and C-terminal fragments in neurons. These results suggest that PS1 is highly expressed and broadly distributed during early stages of neuronal differentiation, consistent with a role for PS1 in neuronal differentiation. Furthermore, the co-localization of PS1 with NFTs and plaque dystrophic neurites implicates a role for PS1 in the diverse pathological manifestations of AD.

Developmental regulation of presenilin-1 processing in the brain suggests a role in neuronal differentiation

Most cases of early-onset familial Alzheimer's disease are caused by mutations in the presenilin genes. Presenilin-1 (PS1) is subject to proteolytic cleavage resulting in the accumulation of N- and C-terminal fragments. In this report, we show that the proteolytic cleavage of PS1 is developmentally regulated in the brain. Low levels of full-length PS1 and higher levels of 30-kDa N-terminal and 20-kDa C-terminal fragments are identified at all developmental stages in the rat brain. However, in the adult brain, additional 36-kDa N-terminal and 14-kDa C-terminal fragments appear and become major PS1 species. Alternative N-terminal PS1 fragments also appear in the adult human brain, but are more heterogenous than in the rat brain. The alternative PS1 fragments are not detected at significant levels in rat or human peripheral tissues that express PS1. The alternative cleavage of PS1 is also detected in primary cultures of rat hippocampal neurons, but not in astrocytes, and is induced by neuronal differentiation. Furthermore, alternative PS1 cleavage is detected in rat PC12 cells and human neuroblastoma SH-SY5Y cells following induction of neuronal differentiation. These results suggest that an alternative pathway of PS1 proteolytic processing is induced in the brain by neuronal differentiation. PS1 may therefore play an important role in brain development and neuronal function, which may relate to the brain-specific pathological effects of PS1 mutations.

Insertion of lysosomal targeting sequences to the amyloid precursor protein reduces secretion of beta A4

The processing of the amyloid precursor protein (APP) was investigated in cells stably expressing different APP hybrid proteins. The cytoplasmic domain of APP was either deleted or replaced by the corresponding domain of the membrane protein TGN38, lamp-1, or LIMPII. The cytosolic domain of TGN38 in the APP molecule did not alter the secretion of beta A4 when compared with the wild-type APP; however, APP associated with the cell surface and the nonamyloidogenic processing of APP were reduced. With the APP molecules carrying the lysosomal targeting signals of lamp-1 or LIMPII, a decrease in the secretion of beta A4 was observed. Cell surface association and nonamyloidogenic processing were also impaired. This suggests increased degradation of APP and thus efficient targeting to the lysosomal system. Cells expressing the Swedish APP variant generated intracellular beta A4 that accumulated after treatment with chloroquine. This effect was more dramatic with APP mutants carrying lysosomal targeting signals than with full-length APP. Our data suggest the existence of an intracellular site of beta A4 generation from where beta A4 is degraded rather than secreted.

Improved electrophoretic separation and immunoblotting of beta-amyloid (A beta) peptides 1-40, 1-42, and 1-43

Beta-amyloid peptides (A beta peptides) form the main protein component of the amyloid deposits found in the brains of Alzheimer's disease (AD) patients. Soluble A beta peptides, which are proteolytic fragments of the amyloid-precursor protein (APP) are constitutively secreted by cells expressing APP during normal metabolism [1] and are also present in human plasma and cerebrospinal fluid [2]. Missense mutations in Codon 717 of the APP gene are responsible for a small percentage of inherited AD cases (FAD) and increase the amount of A beta peptides containing additional carboxy terminal amino acids (A beta 1-42, A beta 1-43) [3, 4]. Recent findings indicate that FAD mutations in the presenilin 1 and 2 genes also increase the amount of these longer A beta peptides [5]. A beta 1-42 polymerizes more rapidly in vitro [6] than A beta 1-40 and has been identified as the major component of the brain amyloid deposits [7-9]. We recently developed a sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) system [10] for the separation of these two peptides. Here we describe a modified version of the original SDS-PAGE procedure, which allows the separation of A beta 1-40, A beta 1-42, and A beta 1-43 for the first time. Detection of the three A beta peptides in the lower ng and pg range is realized by optimized silver staining or immunoblot procedures. These nonradioactive methods may validate results obtained by ELISA procedures used to study the metabolic fate of APP. They may help to define the neurotoxic potential of the longer A beta peptides in relation to their aggregation state.

Distinct processing of endogenous and overexpressed recombinant presenilin 1

The presenilin 1 (PS1) gene has been identified by positional cloning. More than 30 mutations were detected in this gene which cosegregate with Alzheimer's disease (AD). Understanding their role in disease pathogenesis requires a characterization of the PS1 protein. We have generated a set of antibodies against the three major hydrophilic domains of the deduced amino acid sequence. Analyzing cultured cells and brain samples, we identified the endogenous PS1 polypeptide as well as amino- and carboxy-terminal fragments. These metabolites were much more abundant than the full-length molecule, indicating substantial processing. Overexpression of human PS1 markedly increased the full-length polypeptide but hardly altered the amount of the metabolites. Instead, additional proteolytic fragments appeared suggesting a different metabolism of the excess PS1, which may impede studies in transfected cells. Our results indicate a tight regulation of the endogenous PS1 metabolites. PS1 and its fragments are shown to be integral membrane proteins of the endoplasmic reticulum. The mechanisms regulating the generation of the metabolites, their potential function, and role in AD remain to be studied.

The carboxyl termini of beta-amyloid peptides 1-40 and 1-42 are generated by distinct gamma-secretase activities

We have studied the effects of peptide aldehyde protease inhibitors on the secretion of beta-amyloid peptide 1-40 (Abeta(1-40)) and Abeta(1-42) by HEK 293 and COS-1 cells expressing beta-amyloid precursor protein with the Swedish double mutation. A multiphasic SDS-polyacrylamide gel electrophoresis system was used for the discrimination of Abeta(1-40) and Abeta(1-42). Calpain inhibitor I, carbobenzoxyl-Leu-Leu-leucinal, and calpeptin were found to reduce the amount of Abeta(1-40) released into the medium in a dose-dependent manner. The reduction of Abeta(1-40) after treatment with 50 microM calpain inhibitor I or 5 microM carbobenzoxyl-Leu-Leu-leucinal was accompanied by a slight increase of Abeta(1-42) released into the medium. These observations suggest that the cleavages at residues 40 and 42 are accomplished by different enzyme activities.

Amyloid precursor protein truncated at any of the gamma-secretase sites is not cleaved to beta-amyloid

beta A4 secretion occurs upon processing of amyloid protein precursor (APP) by beta-secretase (N-terminus of beta A4) and gamma-secretase (C-terminus). To determine the sequence of these activities and the processing intermediate of beta A4, we expressed several truncated APP molecules in human HEK-293 cells. Immunofluorescence and biotinylation studies indicated that full-length APP or APP lacking the cytosolic domain both were located intracellularly, associated with the cell surface and secreted. APPs truncated after amino acid 40, 42, or 43 of beta A4 were not inserted into cell membranes, were found intracellularly but not on the cell surface, and were efficiently secreted into the culture medium. The secretion of APP truncated at amino acid 40 of beta A4 occurred without proteolytic processing. Neither beta A4 nor P3 (the product of the alpha-secretase) was secreted from any of the APP molecules truncated at the gamma-secretase sites. In sharp contrast to this, when the C-terminal 100 amino acids of APP were expressed (APP truncated at the N-terminus of beta A4), a robust beta A4 secretion was observed. Thus, the C-terminal fragment of APP produced by beta-secretase activity is likely to be the processing intermediate of beta A4.

Electrophoretic separation of betaA4 peptides (1-40) and (1-42)

Different sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) systems designed for the separation of peptides were compared for their usefulness in separating synthetic beta-amyloid peptides betaA4 (1-40) and betaA4 (1-42). Clear resolution was achieved by addition of 8 M urea to the separation gel and use of a multiphasic buffer system employing bicine and sulfate as trailing and leading ions, respectively (bicine/Tris/urea gels). Under these conditions, the longer peptide migrated faster than the one ending at amino acid 40. The usefulness of this SDS-PAGE system for the analysis of betaA4-related peptides generated during cellular metabolism was demonstrated by immunoprecipitation and electrophoretic separation of radiolabeled peptides secreted by cells transfected with amyloid precursor protein cDNAs.

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.

Conformations of synthetic beta peptides in solid state and in aqueous solution: relation to toxicity in PC12 cells

The secondary structures of peptides beta 25-35 (the active toxic fragment) and beta 35-25 (reverse sequence and non-toxic fragment), as well as of the amidated beta (25-35)-NH2 peptide were investigated in aqueous solution and in the solid state by means of Fourier-transformed infrared spectroscopy and circular dichroism spectroscopy. The conformations of the beta 25-35 and beta 35-25 in solid state were identical and contained mostly beta-sheet structures. In solid state the amidated beta (25-35)-NH2 peptide also contained mostly beta-sheet structures. Freshly prepared aqueous solutions of the beta 25-32 (0.5 - 3.8 mM) contained a mixture of beta-sheet and random coil structures. Within 30-60 min incubation at 37 degrees C in water or in phosphate-buffered saline solution (PBS), beta 25-35 was almost fully converted to a beta-sheet structure. Decreasing the temperature from 37 degrees C to 20 degrees C decreased the rate of conversion from random coil to beta-sheet structures, 1-2 h being required for complete conversion. In contrast beta 35-25 in water or in PBS buffer had mostly a random coil structure and remained so for 6 days. The amidated beta(25-35)-NH2 peptide in water (2.7 mM) was also mostly random coil. However, when this peptide (2-2.7 mM) was dissolved in PBS (pH 7.4) or in 140 mM NaCl, a gel was formed and its conformation was mostly beta-sheet. Decreasing the concentration of beta (25-35)-NH2 peptide in 140 mM NaCl aqueous solution from 2 mM to 1 mM or below favored the conversion from beta-sheet structures to random coil structures. The beta 25-35 was toxic to PC12 cells while beta 35-25 was not. The amidated peptide beta (25-35)-NH2 was at least 500-fold less toxic than beta 25-35. Structural differences between these beta peptides in aqueous solutions may explain the difference in their respective toxicities.

[Genetic predisposition for Alzheimer's disease]

Alzheimer's disease comprises senile and presenile dementia. Aetiologically the disease is heterogeneous but has common clinical and pathological characteristics. Furthermore, in patients with a family history of dementia, its incidence is higher than in the general population. In families which inherit early-onset forms of the disease in an autosomal dominant fashion, several apparently pathogenic mutations were identified in three different genes. Dominant hereditary factors are not known for the much more common late-onset forms of Alzheimer's dementia. There are, however, genetic risk factors which contribute to the development of the disease. To date just one of these factors, the apolipoprotein E allele epsilon 4 has been definitively identified. Genetic testing can currently only be done for prognosis of the early onset but not the late onset forms of the disease.

Localization of the 5-hydroxytryptamine2C receptor protein in human and rat brain using specific antisera

The mouse 5-HT2C receptor and its third and fourth (C-terminal) cytoplasmic domain have been expressed as fusion proteins in bacteria. After purification antisera were generated against the fusion proteins. Characterization by immunoblotting using eukaryotic cells expressing the 5-HT2C and 5-HT2A receptors showed that high titer antibodies could be obtained only against the third and fourth cytoplasmic domain but not the entire receptor. Affinity purified antibodies were used to study the location of 5-HT2C receptors in rat and human brain sections. This distribution was compared with the location of 5-HT2C receptor binding sites as determined by [3H]mesulergine, a 5-HT2C receptor radioligand. The antibodies recognized sites in the rat choroid plexus, hippocampus, cerebral cortex, striatum and substantia nigra with a similar distribution as the 5-HT2C binding sites. One antiserum directed against the 5-HT2C receptor C-terminus crossreacted with the human receptor protein in immunoblots. In human brain sections it labelled sites including cerebral cortex, substantia nigra and cerebellum. Our results demonstrate that the antibodies are suitable to identify 5-TH2C receptors in rat and human brain. They visualize a protein distribution which correlates well with the location of the 5-HT2C receptor binding sites as would be expected if affinity states do not influence the binding pattern.