hAPP Mice Research Articles

Antiamyloidogenic and Neuroprotective Functions of Cathepsin B: Implications for Alzheimer's Disease

Alzheimer's disease (AD) may result from the accumulation of amyloid-beta (Abeta) peptides in the brain. The cysteine protease cathepsin B (CatB) is associated with amyloid plaques in AD brains and has been suspected to increase Abeta production. Here, we demonstrate that CatB actually reduces levels of Abeta peptides, especially the aggregation-prone species Abeta1-42, through proteolytic cleavage. Genetic inactivation of CatB in mice with neuronal expression of familial AD-mutant human amyloid precursor protein (hAPP) increased the relative abundance of Abeta1-42, worsening plaque deposition and other AD-related pathologies. Lentivirus-mediated expression of CatB in aged hAPP mice reduced preexisting amyloid deposits, even thioflavin S-positive plaques. Under cell-free conditions, CatB effectively cleaved Abeta1-42, generating C-terminally truncated Abeta peptides that are less amyloidogenic. Thus, CatB likely fulfills antiamyloidogenic and neuroprotective functions. Insufficient CatB activity might promote AD; increasing CatB activity could counteract the neuropathology of this disease.

P1-112: Alzheimer’s disease-related depletions of calcium-dependent proteins in the dentate gyrus may reflect chronic Aβ-induced increases in neuronal excitability

Cognitive deficits in human amyloid precursor protein (hAPP) transgenic mice are tightly linked to the depletion of calcium–dependent proteins in granule cells of the dentate gyrus, and similar molecular alterations have been found in cases with Alzheimer's disease (AD) [PNAS 100: 9572; J. Neurosci. 25: 9686]. Several lines of evidence suggest that these deficits may involve an attenuation of NMDA–dependent signaling pathways that regulate synaptic activity–dependent genes, such as Arc and Fos. Calcium–dependent proteins in granule cells are also decreased in human disorders and mouse models with chronic neuronal hyperexcitability. Here we tested the hypothesis that the depletion of calcium–dependent proteins in the dentate gyrus of hAPP mice is associated with aberrant neuronal excitability. 4– to 6–month–old transgenic mice expressing familial AD–mutant hAPP (line J20) and nontransgenic littermates were first assessed in a battery of behavioral tests. Subsequently, they were challenged by intraperitoneal injection of GABA antagonists to determine their propensity to develop epileptic activity. Behavioral measures and seizure severity were correlated with the levels of calcium–dependent protein levels in the dentate. Compared with nontransgenic controls, hAPP mice were more susceptible to GABA antagonist–induced seizures, although untreated hAPP mice showed no spontaneous seizure activity. The extent of learning and memory deficits of hAPP mice at baseline correlated with seizure severity after injection of the GABA antagonist and with the depletion of calbindin, Arc, and Fos in the dentate gyrus identified postmortem. In nontransgenic mice, glutamate agonists or GABA antagonists caused depletions of calbindin, Arc, and Fos in nontransgenic mice that were qualitatively and quantitatively similar to those observed in untreated hAPP mice. Treatment of hAPP mice with drugs that enhance inhibitory activities prevented some of the Aβ–dependent behavioral deficits. Chronic Aβ–induced increases in neuronal excitability may trigger compensatory mechanisms in the entorhinal–hippocampal network that allow for the survival of granule cells but result in the neuronal depletion of calcium–dependent proteins that are critical for learning and memory. Drugs that block neuronal hyperexcitability and normalize the activity in this network may be of therapeutic benefit in AD.

P3-359: Reduction of mitochondrial superoxide dismutase modulates Alzheimer’s disease-like pathology in human amyloid precursor protein transgenic mice

Alzheimer's disease (AD) results in the accumulation of amyloid–β (Aβ) peptides in the brain, oxidative damage, degeneration of neuronal processes, gliosis, dementia, and death. Whether oxidative damage and mitochondrial decline contribute directly to the characteristic pathology or are a secondary effect of the disease process has not been fully established in vivo. We tested the hypothesis that the mitochondrial antioxidant enzyme superoxide dismutase–2 (Sod2) plays a critical role in defending against Aβ–induced AD–like alterations. To test this hypothesis, we inactivated one Sod2 allele (Sod2+/–) in human amyloid precursor protein (hAPP) transgenic mice. Levels of Sod2 mRNA, protein and activity in Sod2+/– mice were roughly 50% of those in Sod2 wild–type (Sod2+/+) mice. In the presence of normal Sod2 levels, hAPP/Aβ expression resulted in deposition of Aβ predominantly in the brain parenchyma, decreases in levels of the dendritic marker microtubule–associated protein–2 (MAP–2), neuritic dystrophy and behavioral alterations, including deficits in prepulse inhibition of acoustic startle. Sod2 reduction in hAPP mice caused a striking redistribution of amyloid deposits to the cerebrovasculature by middle age and a decrease in parenchymal amyloid plaques by old age, as well as an increased microgliosis and the presence of macrophages with abnormal accumulation of lipid vacuoles near some vessels. Compared with hAPP/Sod2+/+ mice, hAPP/Sod2+/– mice also had an earlier age of onset of behavioral deficits, a greater loss of MAP–2, and a more prominent and widespread neuritic dystrophy, much of which was plaque–independent. Hippocampi of hAPP/Sod2+/– mice had increased DNA binding activity of the injury–responsive transcription factor nuclear factor–κ–B (NFκB). These results suggest that Sod2 protects the aging brain against Aβ–induced impairments. While reductions in Sod2 would be expected to trigger or exacerbate neuronal and vascular pathology in AD, increasing Sod2 activity might be of therapeutic benefit.

O2-05-01: Peripheral expression of neprilysin decreases amyloid deposition in hAPP mice

O2-05-01: Peripheral expression of neprilysin decreases amyloid deposition in hAPP mice

Reduction in mitochondrial superoxide dismutase modulates Alzheimer's disease-like pathology and accelerates the onset of behavioral changes in human amyloid precursor protein transgenic mice.

Alzheimer's disease (AD) is associated with accumulations of amyloid-beta (Abeta) peptides, oxidative damage, mitochondrial dysfunction, neurodegeneration, and dementia. The mitochondrial antioxidant manganese superoxide dismutase-2 (Sod2) might protect against these alterations. To test this hypothesis, we inactivated one Sod2 allele (Sod2(+/-)) in human amyloid precursor protein (hAPP) transgenic mice, reducing Sod2 activity to approximately 50% of that in Sod2 wild-type (Sod2(+/+)) mice. A reduction in Sod2 activity did not obviously impair mice without hAPP/Abeta expression. In hAPP mice, however, it accelerated the onset of behavioral alterations and of deficits in prepulse inhibition of acoustic startle, a measure of sensorimotor gating. In these mice, it also worsened hAPP/Abeta-dependent depletion of microtubule-associated protein 2, a marker of neuronal dendrites. Sod2 reduction decreased amyloid plaques in the brain parenchyma but promoted the development of cerebrovascular amyloidosis, gliosis, and plaque-independent neuritic dystrophy. Sod2 reduction also increased the DNA binding activity of the transcription factor nuclear factor kappaB. These results suggest that Sod2 protects the aging brain against hAPP/Abeta-induced impairments. Whereas reductions in Sod2 would be expected to trigger or exacerbate neuronal and vascular pathology in AD, increasing Sod2 activity might be of therapeutic benefit.

Fyn Kinase Induces Synaptic and Cognitive Impairments in a Transgenic Mouse Model of Alzheimer's Disease

Human amyloid precursor protein (hAPP) transgenic mice with high levels of amyloid-beta (Abeta) develop behavioral deficits that correlate with the depletion of synaptic activity-related proteins in the dentate gyrus. The tyrosine kinase Fyn is altered in Alzheimer's disease brains and modulates premature mortality and synaptotoxicity in hAPP mice. To determine whether Fyn also modulates Abeta-induced behavioral deficits and depletions of synaptic activity-dependent proteins, we overexpressed Fyn in neurons of hAPP mice with moderate levels of Abeta production. Compared with nontransgenic controls and singly transgenic mice expressing hAPP or FYN alone, doubly transgenic FYN/hAPP mice had striking depletions of calbindin, Fos, and phosphorylated ERK (extracellular signal-regulated kinase), impaired neuronal induction of Arc, and impaired spatial memory retention. These deficits were qualitatively and quantitatively similar to those otherwise seen only in hAPP mice with higher Abeta levels. Surprisingly, levels of active Fyn were lower in high expresser hAPP mice than in NTG controls and lower in FYN/hAPP mice than in FYN mice. Suppression of Fyn activity may result from dephosphorylation by striatal-enriched phosphatase, which was upregulated in FYN/hAPP mice and in hAPP mice with high levels of Abeta. Thus, increased Fyn expression is sufficient to trigger prominent neuronal deficits in the context of even relatively moderate Abeta levels, and inhibition of Fyn activity may help counteract Abeta-induced impairments.

Vulnerability of Dentate Granule Cells to Disruption of Arc Expression in Human Amyloid Precursor Protein Transgenic Mice

Activity-induced expression of Arc is necessary for maintenance of long-term potentiation and for memory consolidation. In transgenic (TG) mice with neuronal production of human amyloid precursor protein (hAPP) and hAPP-derived amyloid-beta (Abeta) peptides, basal Arc expression was reduced primarily in granule cells of the dentate gyrus. After exploration of a novel environment, Arc expression in these neurons was unaltered in hAPP mice but increased markedly in nontransgenic controls. Other TG neuronal populations showed no or only minor deficits in Arc expression, indicating a special vulnerability of dentate granule cells. The phosphorylation states of NR2B and ERK1/2 were reduced in the dentate gyrus of hAPP mice, suggesting attenuated activity in NMDA-dependent signaling pathways that regulate synaptic plasticity as well as Arc expression. Arc reductions in hAPP mice correlated with reductions in the actin-binding protein alpha-actinin-2, which is located in dendritic spines and, like Arc, fulfills important functions in excitatory synaptic activity. Reductions in Arc and alpha-actinin-2 correlated tightly with reductions in Fos and calbindin, shown previously to reflect learning deficits in hAPP mice. None of these alterations correlated with the extent of plaque formation, suggesting a plaque-independent mechanism of hAPP/Abeta-induced neuronal deficits. The brain region-specific depletion of factors that participate in activity-dependent modification of synapses may critically contribute to cognitive deficits in hAPP mice and possibly in humans with Alzheimer's disease.

High β-Secretase Activity Elicits Neurodegeneration in Transgenic Mice Despite Reductions in Amyloid-β Levels: IMPLICATIONS FOR THE TREATMENT OF ALZHEIMER DISEASE

Amyloid-beta peptides (Abeta) are widely presumed to play a causal role in Alzheimer disease. Release of Abeta from the amyloid precursor protein (APP) requires proteolysis by the beta-site APP-cleaving enzyme (BACE1). Although increased BACE1 activity in Alzheimer disease brains and human (h) BACE1 transgenic (tg) mice results in altered APP cleavage, the contribution of these molecular alterations to neurodegeneration is unclear. We therefore used the murine Thy1 promoter to express high levels of hBACE1, with or without hAPP, in neurons of tg mice. Compared with hAPP mice, hBACE1/hAPP doubly tg mice had increased levels of APP C-terminal fragments (C89, C83) and decreased levels of full-length APP and Abeta. In contrast to non-tg controls and hAPP mice, hBACE1 mice and hBACE1/hAPP mice showed degeneration of neurons in the neocortex and hippocampus and degradation of myelin. Neurological deficits were also more severe in hBACE1 and hBACE1/hAPP mice than in hAPP mice. These results demonstrate that high levels of BACE1 activity are sufficient to elicit neurodegeneration and neurological decline in vivo. This pathogenic pathway involves the accumulation of APP C-terminal fragments but does not depend on increased production of human Abeta. Thus, inhibiting BACE1 may block not only Abeta-dependent but also Abeta-independent pathogenic mechanisms.

Aggressive amyloidosis in mice expressing human amyloid peptides with the Arctic mutation

The Arctic mutation within the amyloid-beta (Abeta) peptide causes Alzheimer disease. In vitro, Arctic-mutant Abeta forms (proto)fibrils more effectively than wild-type Abeta. We generated transgenic mouse lines expressing Arctic-mutant human amyloid precursor proteins (hAPP). Amyloid plaques formed faster and were more extensive in Arctic mice than in hAPP mice expressing wild-type Abeta, even though Arctic mice had lower Abeta(1-42/1-40) ratios. Thus, the Arctic mutation is highly amyloidogenic in vivo.

Fyn kinase modulates synaptotoxicity, but not aberrant sprouting, in human amyloid precursor protein transgenic mice.

Alzheimer's disease (AD), the most common neurodegenerative disorder, results in progressive degeneration of synapses and aberrant sprouting of axon terminals. The mechanisms underlying these seemingly opposing cellular phenomena are unclear. We hypothesized that Fyn kinase may play a role in one or both of these processes because it is increased in AD brains and because it is involved in synaptic plasticity and axonal outgrowth. We investigated the effects of Fyn on AD-related synaptotoxicity and aberrant axonal sprouting by ablating or overexpressing Fyn in human amyloid precursor protein (hAPP) transgenic mice. On the fyn+/+ background, hAPP/amyloid beta peptide (Abeta) decreased hippocampal levels of synaptophysin-immunoreactive presynaptic terminals (SIPTs), consistent with previous findings. On the fyn-/- background, hAPP/Abeta did not affect SIPTs. SIPT reductions correlated with hippocampal Abeta levels in hAPP/fyn+/+, but not hAPP/fyn-/-, mice suggesting that Fyn provides a critical link between hAPP/Abeta and SIPTs. Furthermore, overexpression of Fyn exacerbated SIPT reductions in hAPP mice. We also found that the susceptibility of mice to hAPP/Abeta-induced premature mortality was decreased by Fyn ablation and increased by Fyn overexpression. In contrast, axonal sprouting in the hippocampus of hAPP mice was unaffected. We conclude that Fyn-dependent pathways are critical in AD-related synaptotoxicity and that the pathogenesis of hAPP/Abeta-induced neuronal alterations may be mechanistically heterogenous.

Neuronal depletion of calcium-dependent proteins in the dentate gyrus is tightly linked to Alzheimer's disease-related cognitive deficits.

Transgenic mice expressing human amyloid precursor proteins (hAPP) and amyloid-beta peptides (Abeta) in neurons develop phenotypic alterations resembling Alzheimer's disease (AD). The mechanisms underlying cognitive deficits in AD and hAPP mice are largely unknown. We have identified two molecular alterations that accurately reflect AD-related cognitive impairments. Learning deficits in mice expressing familial AD-mutant hAPP correlated strongly with decreased levels of the calcium-binding protein calbindin-D28k (CB) and the calcium-dependent immediate early gene product c-Fos in granule cells of the dentate gyrus, a brain region critically involved in learning and memory. These molecular alterations were age-dependent and correlated with the relative abundance of Abeta1-42 but not with the amount of Abeta deposited in amyloid plaques. CB reductions in the dentate gyrus primarily reflected a decrease in neuronal CB levels rather than a loss of CB-producing neurons. CB levels were also markedly reduced in granule cells of humans with AD, even though these neurons are relatively resistant to AD-related cell death. Thus, neuronal populations resisting cell death in AD and hAPP mice can still be drastically altered at the molecular level. The tight link between Abeta-induced cognitive deficits and neuronal depletion of CB and c-Fos suggests an involvement of calcium-dependent pathways in AD-related cognitive decline and could facilitate the preclinical evaluation of novel AD treatments.

Taking Complement Well

Like the baby brother left holding the cookie jar his sister emptied, not all inflammation in Alzheimer's disease (AD) is as blameworthy as appearances suggest. According to new research, certain inflammatory molecules permeate ravaged areas of a diseased brain, but they help more than hurt, at least in mice. The results might allow investigators to determine which inflammatory processes to bolster and which to suppress in the effort to curtail AD. The hallmarks of AD are neuronal degeneration and plaques full of β amyloid, a protein formed by the aberrant processing of β-amyloid precursor protein (APP) (see "Detangling Alzheimer's Disease" ). Evidence conflicts about whether inflammation might cause one or both of these problems. It flourishes in Alzheimer's brains, and anti-inflammatory drugs appear to slow cognitive decline (see McGeer Review ). On the other hand, stimulating the process can slow plaque formation in a mouse model of the disease. Researchers aren't sure how inflammation can swing both ways, but the answer might lie in a biochemical system named the complement cascade that helps the immune system eliminate microbes and infected cells. Some proteins involved in the complement cascade, including one called C3, break down into an active form, C3b, in response to invasion. C3b marks foreign proteins on cells and attracts other complement proteins that punch holes in cell membranes they encounter. The tagging capability also targets proteins for destruction, and this skill might impede AD. The hole punchers, in contrast, perforate willy-nilly and can kill healthy cells--including neurons--in their vicinity. Human Alzheimer's plaques teem with C3b-adorned β amyloid and the cell-drilling proteins, leading some researchers to argue that β amyloid draws C3. Its product summons the piercing proteins, which then kill off nearby neurons. To pin down the complement cascade's role in AD, Wyss-Coray and colleagues thwarted the system in mice. From previous work, Wyss-Coray knew that mice engineered to manufacture human APP (hAPP) accumulate less brain β amyloid if they also overproduce TGF-β, a molecule that promotes inflammation. In the current study, they found that such mice had twice as much C3 in their brains as did hAPP mice without TGF-β. To find out if blocking C3 enhances plaque formation, the team created hAPP mice that crank out a protein called Crry, a complement inhibitor. By 1 year of age, the animals had accumulated two to three times as much brain β amyloid as had mice with complement, and they had lost almost half of their Alzheimer-susceptible neurons. These results indicate that the complement cascade protects brains from β-amyloid deposits. Mouse and human diseases differ, cautions neuroscientist Patrick McGeer of the University of British Columbia in Vancouver, so the current work might not shed light on human AD. For example, human Alzheimer's brains have far more hole punchers than the animal brains do. However, neurobiologist Joseph Rogers of the Sun Health Research Institute in Sun City, Arizona, says McGeer's reservations "don't negate the impact of the study." Whether or not the work relates to the pore-forming proteins, he says, "this study shows [that] if you don't have complement, [β amyloid] builds up. We need to learn to harness this beneficial mechanism while dampening its toxic effects." In AD as in life, sometimes it's nice to get complemented. --Mary Beckman; suggested by Amir Sadighi Akha T. Wyss-Coray, F. Yan, A. H.-T. Lin, J. D. Lambris, J. J. Alexander, R. J. Quigg, E. Masliah, Prominent neurodegeneration and increased plaque formation in complement-inhibited Alzheimer's mice. Proc. Natl. Acad. Sci. U.S.A. , 15 July 2002 [e-pub ahead of print]. [Abstract] [Full text]

Prominent neurodegeneration and increased plaque formation in complement-inhibited Alzheimer's mice.

Abnormal accumulation of beta-amyloid (Abeta) in Alzheimer's disease (AD) is associated with prominent brain inflammation. Whereas earlier studies concluded that this inflammation is detrimental, more recent animal data suggest that at least some inflammatory processes may be beneficial and promote Abeta clearance. Consistent with these observations, overproduction of transforming growth factor (TGF)-beta1 resulted in a vigorous microglial activation that was accompanied by at least a 50% reduction in Abeta accumulation in human amyloid precursor protein (hAPP) transgenic mice. In a search for inflammatory mediators associated with this reduced pathology, we found that brain levels of C3, the central component of complement and a key inflammatory protein activated in AD, were markedly higher in hAPP/TGF-beta1 mice than in hAPP mice. To assess the importance of complement in the pathogenesis of AD-like disease in mice, we inhibited C3 activation by expressing soluble complement receptor-related protein y (sCrry), a complement inhibitor, in the brains of hAPP mice. Abeta deposition was 2- to 3-fold higher in 1-year-old hAPP/sCrry mice than in age-matched hAPP mice and was accompanied by a prominent accumulation of degenerating neurons. These results indicate that complement activation products can protect against Abeta-induced neurotoxicity and may reduce the accumulation or promote the clearance of amyloid and degenerating neurons. These findings provide evidence for a role of complement and innate immune responses in AD-like disease in mice and support the concept that certain inflammatory defense mechanisms in the brain may be beneficial in neurodegenerative disease.

Elevated hepatic and depressed renal cytochrome P450 activity in the Tg2576 transgenic mouse model of Alzheimer's disease.

Recent studies indicate that the Tg2576 transgenic mouse model of Alzheimer's disease [tg(hAPP)] demonstrates disturbances in plasma glucose and neuroendocrine function reminiscent of Alzheimer's disease (AD). Alterations in any one of these systems can have a profound effect on hepatic cytochrome P450 (CYP) expression. Additionally, the recent discovery that amyloid beta 1-42 can induce the expression of CYP reductase in neuronal cultures further suggests that hepatic CYP-related metabolism may be affected by the expression of mutant human amyloid precursor protein in these tg(hAPP) mice. Therefore, the current study was conducted to investigate the activity and protein content of several CYP isoforms in the livers and kidneys of aged (20-month-old) tg(hAPP) mice. tg(hAPP) mice exhibit significant elevations in hepatic CYP2B, CYP2E1-, CYP3A- and CYP4A-associated activities and CYP4A immunoreactive protein compared with wild-type. In contrast to the liver, a significant depression in renal CYP2E1- and CYP4A-associated activities were demonstrated in tg(hAPP) mice. The presence of the mutant hAPP protein was detected in the brain, kidney and livers of tg(hAPP) mice.

Brain Proteins Point to Link Between Parkinson’s, Alzheimer’s

Back to table of contents Previous article Next article Clinical & Research NewsFull AccessBrain Proteins Point to Link Between Parkinson’s, Alzheimer’sJim RosackJim RosackSearch for more papers by this authorPublished Online:19 Oct 2001https://doi.org/10.1176/pn.36.20.0023aScientists from the University of California at San Diego (UCSD) have discovered that two of the proteins associated with Alzheimer’s disease and Parkinson’s disease closely interact to enhance each other’s distinct degenerative effects.In an early-edition online report posted September 25 on the Proceedings of the National Academy of Sciences Web site, the team suggested that pharmacological therapies that block the production or accumulation of either protein may have broader therapeutic effects than previously thought.Although Alzheimer’s disease (AD) and Parkinson’s disease (PD) are distinct neurological disorders, previous studies have indicated that as many as 25 percent of patients with AD develop PD-like symptoms, and some PD patients develop signs of AD as well.Some 4 million patients have AD in the United States, and more than 1 million are currently being treated for PD. Each disorder appears to have a specific group of neurons within specific areas of the brain that it targets. In these specific brain regions, AD is known to be associated with the overproduction or abnormal buildup of the protein beta amyloid into pathological brain changes known as amyloid plaques and PD with a similar pathological accumulation of the protein alpha-synuclein into Lewy bodies. Familial forms of each disease have been associated with genetic mutations leading to the overaccumulation of these proteins.In the current report, Eliezer Masliah, M.D., a professor of neurology and pathology at UCSD, led a group of researchers from UCSD and the Gladstone Institute of Neurological Disease and department of neurology at the University of California at San Francisco in developing strains of genetically engineered mice that produce the two human proteins. Their work was funded by the National Institutes of Health.One strain of mice was altered to produce human amyloid precursor protein (hAPP)—the building block of beta amyloid—while a second strain was altered to produce human alpha-synuclein (hSYN). A third strain of mice was engineered to overproduce both proteins.Reports of human cases with clinical and neuropathological features of both AD and PD led the researchers to hypothesize that hSYN and hAPP have “distinct, as well as convergent, pathogenic effects on the integrity and function of the brain,” according to the report. The team suspected that hSYN might affect motor functions more than cognitive ones, while hAPP has the opposite affect. In addition, they believed, the two proteins might interact more directly by engaging in synergistic neurodegenerative pathways.To explore the possible connection between the two disease processes, Masliah’s team studied the three strains of mice. They followed motor function and ability to learn and used extensive postmortem analysis to determine the end results of the two genetic alterations, either singly or in combination.The researchers report that mice that produced only one of the proteins developed the symptoms of the disease tied to that respective protein. But when both proteins were produced in the same mouse, the Alzheimer’s-like symptoms of the hAPP mice—including an impaired ability to learn and the pathological degeneration of particular brain cells—were exacerbated by the production of hSYN. The Parkinson-like motor deficits of the hSYN mice also tended to develop sooner in the mice that also expressed hAPP.An unexpected finding, according to the report, was a direct enhancement of the accumulation of hSYN in brain cells with increasing levels of beta amyloid. The researchers say that because the proteins appear to interact to accelerate and exacerbate the symptoms of their respective diseases, medications could be developed that prevent the accumulation of hAPP or hSYN, leading to benefits for a wider spectrum of neurodegenerative disorders than has previously been thought.An abstract of the study, “Beta-Amyloid Peptides Enhance Alpha-Synuclein Accumulation and Neuronal Deficits in a Transgenic Mouse Model Linking Alzheimer’s Disease and Parkinson’s Disease,” is posted on the Web at www.pnas.org/cgi/content/abstract/211412398v1. ▪ Proc. Natl. Acad. Sci. USA 2001 98 12245 ISSUES NewArchived

A learning deficit related to age and beta-amyloid plaques in a mouse model of Alzheimer's disease.

Mice that overexpress the human mutant amyloid precursor protein (hAPP) show learning deficits, but the apparent lack of a relationship between these deficits and the progressive beta-amyloid plaque formation that the hAPP mice display is puzzling. In the water maze, hAPP mice are impaired before and after amyloid plaque deposition. Here we show, using a new water-maze training protocol, that PDAPP mice also exhibit a separate age-related deficit in learning a series of spatial locations. This impairment correlates with beta-amyloid plaque burden and is shown in both cross-sectional and longitudinal experimental designs. Cued navigation and object-recognition memory are normal. These findings indicate that A beta overexpression and/or A beta plaques are associated with disturbed cognitive function and, importantly, suggest that some but not all forms of learning and memory are suitable behavioural assays of the progressive cognitive deficits associated with Alzheimer's-disease-type pathologies.

High-Level Neuronal Expression of Aβ1–42in Wild-Type Human Amyloid Protein Precursor Transgenic Mice: Synaptotoxicity without Plaque Formation

Amyloid plaques are a neuropathological hallmark of Alzheimer's disease (AD), but their relationship to neurodegeneration and dementia remains controversial. In contrast, there is a good correlation in AD between cognitive decline and loss of synaptophysin-immunoreactive (SYN-IR) presynaptic terminals in specific brain regions. We used expression-matched transgenic mouse lines to compare the effects of different human amyloid protein precursors (hAPP) and their products on plaque formation and SYN-IR presynaptic terminals. Four distinct minigenes were generated encoding wild-type hAPP or hAPP carrying mutations that alter the production of amyloidogenic Abeta peptides. The platelet-derived growth factor beta chain promoter was used to express these constructs in neurons. hAPP mutations associated with familial AD (FAD) increased cerebral Abeta(1-42) levels, whereas an experimental mutation of the beta-secretase cleavage site (671(M-->I)) eliminated production of human Abeta. High levels of Abeta(1-42) resulted in age-dependent formation of amyloid plaques in FAD-mutant hAPP mice but not in expression-matched wild-type hAPP mice. Yet, significant decreases in the density of SYN-IR presynaptic terminals were found in both groups of mice. Across mice from different transgenic lines, the density of SYN-IR presynaptic terminals correlated inversely with Abeta levels but not with hAPP levels or plaque load. We conclude that Abeta is synaptotoxic even in the absence of plaques and that high levels of Abeta(1-42) are insufficient to induce plaque formation in mice expressing wild-type hAPP. Our results support the emerging view that plaque-independent Abeta toxicity plays an important role in the development of synaptic deficits in AD and related conditions.

Elimination of the Class A Scavenger Receptor Does Not Affect Amyloid Plaque Formation or Neurodegeneration in Transgenic Mice Expressing Human Amyloid Protein Precursors

The class A scavenger receptor (SR) is expressed on reactive microglia surrounding cerebral amyloid plaques in Alzheimer's disease (AD). Interactions between the SR and amyloid beta peptides (Abeta) in microglial cultures elicit phagocytosis of Abeta aggregates and release of neurotoxins. To assess the role of the SR in amyloid clearance and Abeta-associated neurodegeneration in vivo, we used the platelet-derived growth factor promoter to express human amyloid protein precursors (hAPPs) in neurons of transgenic mice. With increasing age, hAPP mice develop AD-like amyloid plaques. We bred heterozygous hAPP (hAPP(+/-)) mice that were wild type for SR (SR(+/+)) with SR knockout (SR(-/-)) mice. Crosses among the resulting hAPP(+/-)SR(+/-) offspring yielded hAPP(+/-) and hAPP(-/-) littermates that were SR(+/+) or SR(-/-). These second-generation mice were analyzed at 6 and 12 months of age for extent of cerebral amyloid deposition and loss of synaptophysin-immunoreactive presynaptic terminals. hAPP(-/-)SR(-/-) mice showed no lack of SR expression, plaque formation, or synaptic degeneration, indicating that lack of SR expression does not result in significant accumulation of endogenous amyloidogenic or neurotoxic factors. In hAPP(+/-) mice, ablation of SR expression did not alter number, extent, distribution, or age-dependent accumulation of plaques; nor did it affect synaptic degeneration. Our results do not support a critical pathogenic role for microglial SR expression in neurodegenerative alterations associated with cerebral beta amyloidosis.

Levels and Alternative Splicing of Amyloid β Protein Precursor (APP) Transcripts in Brains of APP Transgenic Mice and Humans with Alzheimer's Disease

Abnormal expression of human amyloid precursor protein (hAPP) gene products may play a critical role in Alzheimer's disease (AD). Recently, a transgenic model was established in which platelet-derived growth factor (PDGF) promoter-driven neuronal expression of an alternatively spliced hAPP minigene resulted in prominent AD-type neuropathology (Games, D., Adams, D., Alessandrini, R., Barbour, R., Berthelette, P., Blackwell, C., Carr, T., Clemens, J., Donaldson, T., Gillespie, F., Guido, T., Hagopian, S., Johnson-Wood, K., Khan, K., Lee, M., Leibowitz, P., Lieberburg, I., Little, S., Masliah, E., McConlogue, L., Montoya-Zavala, M., Mucke, L., Paganini, L., and Penniman, E. (1995) Nature 373, 523-527). Here we compared the levels and alternative splicing of APP transcripts in brain tissue of hAPP transgenic and nontransgenic mice and of humans with and without AD. PDGF-hAPP mice showed severalfold higher levels of total APP mRNA than did nontransgenic mice or humans, whereas their endogenous mouse APP mRNA levels were decreased. This resulted in a high ratio of mRNAs encoding mutated hAPP versus wild-type mouse APP. Modifications of hAPP introns 6, 7, and 8 in the PDGF-hAPP construct resulted in a prominent change in alternative splice site selection with transcripts encoding hAPP770 or hAPP751 being expressed at substantially higher levels than hAPP695 mRNA. Frontal cortex of humans with AD showed a subtle increase in the relative abundance of hAPP751 mRNA compared with normal controls. These data identify specific intron sequences that may contribute to the normal neuronspecific alternative splicing of APP pre-mRNA in vivo and support a causal role of hAPP gene products in the development of AD-type brain alterations.