- Email: [email protected]
Transgenic mouse models of Alzheimer's disease
TRENDS in Genetics, Vol.17 No.10, October 2001
A TRENDS Guide to Mouse Models of Human Diseases
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Review
Transgenic mouse models of Alzheimer’s disease Brian J. Hock, Jr and Bruce T. Lamb Recent advances in the understanding of the genetic basis of Alzheimer’s disease have enabled the production of transgenic mouse models of the disease. Utilizing both cDNA- and genomic-based approaches, these mouse models for Alzheimer’s disease have already provided valuable insights into the pathogenesis of the disease and potential therapeutic interventions. Alzheimer’s disease (AD), the most common dementing disorder of later life, is a major cause of disability and death in the elderly1. Current estimates indicate that up to 10% of the population over the age of 65, and 50% over the age of 80, are afflicted by AD. Tragically, the progression of the disease is lengthy and there is currently no effective treatment. As people born during the population boom of the late 1940s and 1950s reach their 60s and 70s, AD will become a health care problem of catastrophic proportions. The cardinal clinical feature of AD is memory impairment that significantly worsens upon disease progression. A typical neuropsychological profile of an individual with mild to moderate AD includes recent declarative (explicit) memory impairments, deficits in problem solving, judgement and abstract reasoning, reduction in verbal fluency, and, often, language deficits. The memory loss exhibited in AD is dependent on the hippocampal system [dentate gyrus, cornu ammonis (CA)1–CA3, and rhinal cortices]. As the disease progresses, global amnesia, dependent on other cortical areas, debilitates the individual. Although dementia is the defining clinical feature of AD, a definitive diagnosis requires post-mortem examination of brain tissue for the presence of distinctive AD histopathology, namely filamentous inclusions in neuronal cell bodies (neurofibrillary tangles) and processes (neurites), extracellular deposits of the β-amyloid (Aβ) protein in senile plaques (Fig. 1) and within the walls of leptomeningeal/cerebral vessels, and synaptic and neuronal cell loss. At present, the relationship between these neuropathological observations and the onset of dementia remains quite controversial. In large part, this reflects the variability in the presentation of AD in terms of the length, severity and age-related progression of the disease. Accumulating evidence suggests that the development and progression of AD is subject to a wide variety of both environmental and genetic modifiers.
Genetics of Alzheimer’s disease Genetic investigations have demonstrated that AD is a heterogeneous disorder with several known etiologies: dosage imbalance for chromosome 21, as occurs in Down’s syndrome (DS); mutations in the gene encoding amyloid precursor protein (APP) on chromosome 21, the gene encoding presenilin-1 (PS1) on chromosome 14, and the gene encoding presenilin-2 (PS2) on chromosome 1, as occurs in autosomal dominant early-onset familial AD (FAD); and inheritance of distinct α-2 macroglobulin (A2M) and apolipoprotein E (ApoE) gene alleles on chromosomes 12 and 19, respectively, as significant genetic risk factors for late-onset FAD. Recently, numerous biochemical, biophysical and immunohistochemical studies have provided strong evidence that many of the genetic forms of AD share common pathogenic mechanisms that involve alterations in the metabolism of Aβ peptides. The predominant form of Aβ is a 39–43 amino acid peptide comprising 28 amino acids of the ectodomain and 11–15 amino acids of the adjacent transmembrane domain of APP (Fig. 2). The gene encoding APP encompasses 18 exons spanning 300 kb of DNA, giving rise to at least four tissue-specific alternatively spliced transcripts that encode proteins of 695, 714, 751 and 770 amino acids. APP, a type-I integral membrane glycoprotein of unknown function, matures through the constitutive secretory pathway. APP is preferentially processed by α-secretase cleavage, releasing the APP ectodomain (APPsα) into the extracellular space with retention of an intracellular C-terminal fragment (CTFα), which prevents the formation of full-length Aβ. Cleavage by β-secretase at the N-terminus of Aβ, however, releases a C-terminal APP fragment containing Aβ (CTFβ), which when followed by γ-secretase cleavage releases fulllength Aβ (Fig. 2) through pathways that remain obscure. Finally, APP can also be internalized and degraded through endosomal–lysosomal pathways.
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Brian J. Hock, Jr and Bruce T. Lamb* Dept of Genetics, Case Western Reserve University, University Hospitals of Cleveland, 10900 Euclid Avenue, Cleveland, OH 44106-4955, USA. *e-mail: [email protected]
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(a)
Figure 1. Deposition of Aβ in human Alzheimer’s disease and transgenic mouse brain Brain sections from (a) an end-stage sporadic human Alzheimer’s disease (AD) case and (b) a 14-month-old male TgR1.40 homozygous mutant APP YAC transgenic mouse, were stained with 6E10 — a human-specific monoclonal antibody for Aβ. Note the similarity of the Aβ deposits between human and mouse. Scale bars, 0.125 mm.
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(b)
Although the mechanisms and cell types involved in the generation, turnover and transport of Aβ and the formation of amyloid deposits in vivo remain poorly defined, several recent studies have provided insight into the secretases that cleave APP (Ref. 2). Two members of the disintegrin and metalloprotease family (ADAM), the membranebound tumor necrosis factor-α converting enzyme (TACE or ADAM17) and ADAM10, have been directly implicated in the α-secretase cleavage of APP. At present, however, it remains unclear which of these α-secretase activities contribute to the normal in vivo processing of APP and/or to the altered processing that results in AD. Recently, the enzymes responsible for β-secretase cleavage of APP were identified and termed the β-site APP cleaving enzyme (BACE) 1 and 2 (Ref. 2). BACE1 and BACE2 are transmembrane aspartyl proteases that can cleave both at the N-terminus of full-length Aβ and several locations within Aβ. Finally, although the exact identity of the γ-secretase remains a mystery, recent evidence suggests that PS1 normally indirectly or directly facilitates the proteolysis of APP by γ-secretase. The observation that older individuals with DS invariably develop AD histopathology suggests that dosage imbalance and the resulting overexpression of APP and Aβ leads to AD. In addition, in vitro transfection approaches, as well as studies using transgenic mice, have provided substantial insights into the mechanisms by which the FAD mutations affect the processing of APP and the production of Aβ (Fig. 2)1.Transfection studies have demonstrated that cells that express APP harboring the K670N/M671K or the D692G FAD substitutions, produce higher levels of CTFβ and secrete higher levels of all Aβ peptides as compared to cells expressing wildtype (wt) constructs through altered β-secretase cleavage. Cells that express APP harboring the I716V or the 717 (V717I, V717F, V717G and V717L) FAD substitutions do not secrete higher levels of Aβ, but rather release greater amounts of full-length Aβ peptides extending to residue 42 (Aβ1–42). Cells expressing the T714I mutant secrete higher levels of Aβ1–42, lower levels of Aβ1–40 and increased levels of N-terminally truncated Aβ ending at residue 42 (AβX–42), whereas the V715M mutation specifically increases the level of AβX–42. Taken together with immunocytochemical studies of human
TRENDS in Genetics, Vol.17 No.10, October 2001
AD and DS cortex, as well as biophysical studies of Aβ, these results suggest that Aβ ending at residue 42 (either Aβ1–42 or AβX–42) is pathogenic, although the exact in vivo pathways that regulate these processes remain unclear. A majority of early-onset FAD pedigrees are linked to mutations in the PS1 gene on chromosome 14 and a much smaller percentage to mutations in the closely related PS2 gene on chromosome 1. The PS1/PS2 genes are highly conserved (67% identical in primary sequence) and both encompass 13 exons covering the ~70–100 kb that encode multiple alternatively spliced transcripts, which, in turn, give rise to polypeptides of 431–467 amino acids for PS1, and 415–448 amino acids for PS2. One hint about the pathological consequence of presenilin mutations comes from studies of Aβ metabolism, demonstrating an increased secretion of Aβ1–42 in cultured fibroblasts, plasma and brains from individuals with PS1 mutations, as well as transfected cell lines and transgenic mice.These studies strongly suggest that PS1 and PS2 variants could influence APP metabolism to promote the production of highly amyloidogenic Aβ1–42 peptides. Finally, current evidence suggests that both the late-onset FAD genes ApoE and A2M regulate the deposition and/or clearance of Aβ through pathways that remain obscure. Transgenic mouse models of Alzheimer’s disease One of the major difficulties in studying the relationship between the production and deposition of Aβ, the onset of dementia and the neuritic abnormalities, synaptic dysfunction and neuronal cell death that occurs in AD, has previously been the paucity of mouse models of the disease. Transgenic experiments can test directly whether the overexpression of wt or mutant APP, PS1 or PS2 genes leads to AD-type abnormalities and thus provide insight into the molecular mechanism(s) of disease. Over the past several years, numerous groups have generated mice with various features of AD using both cDNA- and genomicbased approaches, a number of which are summarized here with additional details provided in Table 1. cDNA-based transgenics Games et al.3 introduced a complex transgenic construct consisting of the platelet-derived growth factor (PDGF) β-chain promoter and a human APP mini-gene containing the APP Val717→Phe FAD mutation in an APP cDNA with portions of APP introns 6–8, enabling alternative splicing of exons 7 and 8. In the one transgenic line described (termed PD-APP) in a hybrid C57BL/6-DBA/2-Swiss Webster background, Southern blots disclosed ~40 copies of the transgene inserted at a single site and transmitted in a stable fashion. Levels of human APP mRNA were significantly greater than endogenous App transcripts; the three major splice variants of APP mRNA were demonstrable, albeit at dramatically abnormal ratios, and levels of APP were ~10-fold higher than endogenous mouse App. At
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eight months of age, Aβ deposits were observed in the hippocampus, corpus callosum and cerebral cortex; neurofibrillary tangles were not present (and not present in any of the other mouse models of AD). No significant neuronal loss was observed4, but it was recently reported that PD-APP mice were impaired in a novel spatial memory task in an age-dependent manner that correlated with the presence of Aβ deposits5. In a pioneering set of studies, immunization of either young or aged PD-APP mice with Aβ peptide resulted in a significant delay or reduction, respectively, of AD-like neuropathologies6. Hsiao et al.7 created transgenic mice (termed Tg2576) that express the 695 amino acid form of human APP containing the K670N/M671L FAD mutation, placed under the transcriptional control of the hamster prion gene promoter. In the single transgenic line described in a hybrid C57BL/6-SJL background, the levels of transgene-encoded APP were ~5.5-fold higher than endogenous mouse App, and at 9–11 months of age Aβ deposits were observed in numerous brain regions; neurofibrillary tangles were not present.These animals also developed behavioral disorders, including reduced spontaneous alternation performance in a Y-maze and increased escape latency in the water maze, although they failed to exhibit significant neuronal loss7,8. Recent data demonstrated that vaccination of Tg2576 mice with Aβ1–42 peptide reduces amyloid deposition and leaves spatial memory intact, compared with unvaccinated age-matched transgenics and controls9. Finally, the phenotypes observed in these mice are highly dependent on genetic background with embryonic lethality associated with backcrossing into certain inbred mouse strains10. Novartis Pharmaceuticals (Basel, Switzerland) developed two transgenic mouse models of AD expressing the 751 amino acid form of human APP in a hybrid C57BL/ 6-DBA/2 background11. In transgenic line TgAPP23, containing an APP cDNA with the K670N/M671L FAD mutation under the control of the murine Thy-1 promoter, the transgene was expressed sevenfold higher than endogenous App and the animals developed parenchymal and vascular Aβ deposits at six months of age. At 14 months of age, these mice displayed between 14 and 25% hippocampal CA1 neuronal loss that correlated with Aβ deposition12. Transgenic line TgAPP14/22, containing an APP cDNA with both the K670N/M671L and V717I mutations under the control of the human Thy-1 promoter, expressed the transgene twofold higher than endogenous App and developed Aβ deposits much later in life. Thus far, behavioral measurements have not been reported for these animals. Borchelt et al.13,14 developed an APP transgenic mouse (termed TgC3-3) using a mouse APP 695-cDNA with the L670N/M671L mutation and humanized Aβ (driven by the murine prion promoter) in a hybrid C3H/HeC57BL/6 background. These mice expressed the APP transgene at twofold higher levels than endogenous App, and developed Aβ deposits at 18 months of age. A similar cDNA transgenic model (termed TgCRND8) was recently
A TRENDS Guide to Mouse Models of Human Diseases
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Lipid bilayer
APP 751/770 KPI insert
Aβ
NH2 1
COOH 695 β-Secretase
α-Secretase
γ-Secretase (Aβ1– 42)
VKM DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA TVIVITLVMLK
K670N/M671L
A692G
T714I V715M I716V
L723P V717I V717G V717F V717L
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reported that contains an APP 695-cDNA with both the K670N/M671L and V717F mutations (under the control of the Syrian hamster prion promoter) in a hybrid C3H/He-C57BL/6 background15,16. The APP transgene was expressed at fivefold higher levels than endogenous APP, and Aβ deposits were observed as early as three months of age. It was also reported that immunization with Aβ1–42 reduces plaque load and behavioral impairments in TgCRND8 compared with nonimmunized Tg animals and controls. These mice also exhibit premature death that is dependent upon genetic background15. Van Leuven and colleagues17 generated a transgenic line (termed TgAPP/Ld/2) that contains the murine Thy-1 promoter driving expression of an APP-695 cDNA with the V717I mutation in the FVB/N background. The transgene was expressed at 25-fold higher levels than endogenous App mRNA, and fibrillar Aβ deposits develop from 13 months of age.These mice also show memory impairments in the water maze at three and six months of age. In addition, these mice exhibit premature death (72% at six months) and increased aggressiveness17–20. Genomic-based transgenics Although the cDNA-based transgenic mouse models of AD provided evidence that transgenic mice can develop Aβ deposits, it is unclear how the observed Aβ deposition relates to that observed in AD. Indeed, each of these cDNA-based transgenic studies relies on certain basic assumptions about the disease process with regard to the APP cDNA (695, 751 or 770 amino acid form) and the promoter (neuronal-specific, glial-specific, CNS-specific, brain region-specific, etc.) utilized to drive the temporal and spatial expression of the transgene. Because of these assumptions, it remains unclear how the formation of Aβ deposits in these transgenic mice will be related to the onset of behavioral impairments, the formation of neurofibrillary tangles, the neuronal cell loss and other phenotypes observed in individuals with AD. By contrast, the genomic-based approaches utilize the native human or mouse APP genomic sequence that contains the transcriptional regulatory elements required for proper spatial and
Figure 2. Schematic of the 695 isoform of amyloid precursor protein (APP) The figure depicts the site of the APP 751/770 Kunitz-type serine protease inhibitor (KPI) insert, highlighting an enlarged view of Aβ (purple box) and lipid bilayer (turquoise box) with the corresponding amino acid sequence. The α-, β- and γ-secretase cleavage sites are identified with arrows above the enlarged Aβ peptide with the various FAD mutations designated below. The FAD APP mutations are as follows: K670N/M671L is the Swedish mutation with the lysine→asparagine and methionine→leucine substitutions; A692G is the Flemish mutation (alanine→glycine); T714I is the Austrian mutation (threonine→isoleucine); V715M is the French mutation (valine→methionine); and I716V is the Florida mutation (isoleucine→valine); The 717 mutations are as follows: London, valine→isoleucine; valine→glycine; Indiana, valine→phenylalanine; and valine→leucine. L723P is the Australian mutation (leucine→proline). Transgenic mice with the Swedish, London or Indiana mutations are described in the review.
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TRENDS in Genetics, Vol.17 No.10, October 2001
Table 1. APP transgenic mouse models of Alzheimer’s diseasea Transgenic line
Mouse strainb
Approachc
Mutationd
Promoter e
APP expressionf
PD-APP
Swiss Webster, C57BL/6
Modified cDNA (695, 751, 770)
V717F
PDGF-β
10x
Tg2576
C57BL/6-SJL
cDNA (695)
Swedish
Hamster PrP
5–6x
Tg-APP23
C57BL/6-DBA/2, C57BL/6 cDNA (751)
Swedish
Murine Thy-1
7x
TgC3-3
C57BL/6-C3H
cDNA (695)
Swedish
Murine PrP
2x
TgCRND-8
C57BL/6-C3H
cDNA (695)
Swedish and V717I
Hamster PrP
5x
TgAPP/Ld/2
FVB/N
cDNA (695)
V717I
Murine Thy-1
25x (RNA)
TgAPP/Sw/1
FVB/N
cDNA (695)
Swedish
Murine Thy-1
7x (RNA)
TgAPPSwe-KI
CD-1
Genomic (695, 751, 770)
Swedish
Mouse App
0.5x
TgR1.40-YAC
C57BL/6, DBA/2,
Genomic (695, 751, 770)
Swedish
Human APP
Hemi/homo 2–3x/4–6x
129S3/SvIm, A aAbbreviations:
APP, amyloid precursor protein; n.r., not reported; PDGF, platelet-derived growth factor. strain of mouse or congenic generated. Designations separated by a hyphen represent hybrid genetic backgrounds. ccDNA or genomic-based [yeast artificial chromosome (YAC) or knock-in (KI)] transgenic approach, including the various APP cDNAs expressed from the construct (695, 751 or 770 amino acids). dAPP FAD mutation [V717F, V717I or Swedish (K670N/M671L)]. ePromoter utilized in transgenic construct. fLevels of APP expression (protein levels relative to endogenous mouse App unless otherwise specified). gAge of onset of Aβ deposits (earliest timepoint). hAcceleration of Aβ deposition of PS-1 FAD mutations. bInbred
temporal expression, with appropriate splice donor and acceptor sites needed to generate the entire spectrum of alternatively spliced transcripts (i.e. 695, 751 and 770). Thus, the genomic-based approach, unlike the cDNA-based transgenic approaches described above, make few assumptions about the molecular pathogenesis of AD, both in terms of the regional and temporal specificity of the disease. Utilizing a gene-targeting strategy, investigators at Cephalon (West Chester, PA, USA) humanized the mouse Aβ and introduced the K670N/M671L FAD mutation into a single transgenic line (termed TgAPPSwe-KI) in an outbred CD-1 background21. This unique approach resulted in the expression of human APP at 0.5-fold the level of endogenous App. Increased brain levels of human CTFβ and Aβ and decreased levels of APPsα were observed in a dosage-dependent manner. Neuropathology and behavior have not been described thus far. Another genomic-based approach relies on the introduction of the entire human AD genes cloned on yeast artificial chromosomes (YACs) into the germline of mice. Lamb et al.22,23 generated several wt and mutant APP YAC transgenic lines, including one (termed TgR1.40) with the entire 300 kb human APP gene harboring the K670N/M671L mutation on a hybrid C57BL/6-129/Sv background. Human-derived transgene products were expressed at two- to threefold the level of endogenous mouse App in the hemizygous state, and four- to sixfold in the homozygous state. Hemizygous and homozygous
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TgR1.40 animals develop Aβ deposits (primarily Aβ1–42) at 24–26 months and 14–16 months of age, respectively24. Recently, aged homozygous TgR1.40 mice were shown to exhibit behavioral impairments on spontaneous alternation, water maze retention and interference on the retention of an odor-guided task (Brian J. Hock, unpublished results).The TgR1.40 mice have recently been backcrossed into four genetic backgrounds including A, C57BL/6, DBA/2 and 129S3/SvIm (Bruce T. Lamb, unpublished results). Modifier genes The role of specific AD modifier genes has been tested in the APP transgenic mice described above by mating to other genetically modified mice. For example, several groups have demonstrated that transgene-derived expression of PS1 with FAD mutations results in elevation of Aβ1–42 and an earlier age of Aβ deposition in both cDNAand genomic-based APP transgenic mice14,24–27. Other genes that have been examined through crosses to APP transgenic mice include the different human ApoE isoforms28, α-1-antichymotrypsin29,30 and transforming growth factor (TGF) β131. Concluding remarks AD is a genetically heterogeneous disorder with many documented risk factors. After age, genetics is the second biggest risk factor. Utilizing information gleaned from
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Aβ deposits (age of onset)g
Acceleration by PS-1h
Neuritesi
NFT elementsj
Neuronal cell lossk
Behavioral impairmentsl
Immunizationm
Refs
6 months
n.r.
Yes
AT8 positive
Not detectable
Yes
Yes
3–6
9–12 months
Yes
Yes
AT8 positive
Not detectable
Yes
Yes
7–10
6 months
Yes
Yes
AT8 positive
25% in CA1 at
n.r.
n.r.
11,12
14 months 18 months
Yes
Yes
n.r.
n.r.
Yes
Yes
13,14
3–4 months
Yes
Yes
n.r.
Not detectable
Yes
Yes
15,16
13–18 months
Yes
Yes
AT8 positive
No overt loss
Yes
Yes
17–20
18 months
Yes
Yes
AT8 positive
No overt loss
n.r.
n.r.
19
n.r.
Yes
n.r.
n.r.
n.r.
n.r.
n.r.
21
Hemi/homo; 22 months/
Yes
Yes
AT8 positive
n.r.
Yes
n.r.
22–24
14 months iPresence
of neurites surrounding the Aβ deposits identified by silver staining, immunostaining and/or electron microscopy. Inflammation was identified in all mouse models by presence of microglia around the Aβ deposits and various inflammatory cytokines, as well as astrocytosis. jAlthough no neurofibrillary tangles (NFTs) have been observed in any APP transgenic mice, many exhibit the presence of hyperphosphorylated tau as documented by staining with phosphorylation-specific tau antibody AT8. kNeuronal cell loss as documented by cell counts in various brain regions. lBehavioral impairments observed in transgenic mice as documented by the Morris water maze (and modifications thereof), spatial alternation and other specific behavioral tasks. mImmunization performed with either Aβ peptide or anti-Aβ antibodies.
genetic studies of AD, various cDNA-based and genomicbased transgenic mouse models of AD have been developed. At present, the similarities and differences of the various mouse models remain unclear as they all have been developed with different APP cDNAs, FAD mutations, promoters and genetic background of the mouse strain. Future studies to examine the common and unique characteristics of these models in defined genetic backgrounds will be vital as they are currently being widely used by both academic and industry laboratories to examine the effects of therapeutic, environmental and genetic modification of phenotypes. Finally, although none of the transgenic models of AD recapitulate all of the features of the disease, these gene-based transgenic mouse models for AD have already provided valuable insights into the pathogenesis of the disease and potential therapeutic interventions that could prove efficacious in the treatment of AD. Acknowledgements We would like to thank M. Chiocco, E.H. Lehman and K. Pimpis for critical review of the manuscript. The Lamb laboratory is supported by NIH grant AG14451 and the Alzheimer’s Association grant IIRG-99-15157 (B.T.L.), by the NRSA Training Grant 5 T32 AG00105 and NIA 1 F32 AG05875-01A1 (B.J.H.) as well as support from the University Alzheimer Center (AG08012) and the Ireland Cancer Center (CA43703).
References 1 Selkoe, D.J. (2001) Alzheimer’s disease: genes, proteins, and therapy. Physiol. Rev. 81, 741–766 2 Nunan, J. and Small, D.H. (2000) Regulation of APP cleavage by α-, β- and γ-secretases. FEBS Lett. 483, 6–10 3 Games, D. et al. (1995) Alzheimer-type neuropathology in transgenic mice overexpressing V717F β-amyloid precursor protein. Nature 373, 523–527 4 Irizarry, M.C. et al. (1997) Aβ deposition is associated with neuropil change, but not overt neuronal loss in human amyloid precursor V717F (PD-APP) transgenic mice. J. Neurosci. 17, 7053–7059 5 Chen, G. et al. (2000) A learning deficit related to age and β-amyloid plaques in a mouse model of Alzheimer’s disease. Nature 408, 975–979 6 Schenk, D. et al. (1999) Immunization with amyloid-β attenuates Alzheimer-disease-like pathology in the PDAPP mouse. Nature 400, 173–177 7 Hsiao, K. et al. (1996) Correlative memory deficits, Aβ elevation and amyoid plaques in transgenic mice. Science 274, 99–102 8 Irizarry, M.C. et al. (1997) Tg (HuAPP695.K670N-M671L) 2576 mice develop age-related deposits and neuropil abnormalities, but no neuronal loss in CA1. J. Neuropathol. Exp. Neurol. 56, 965–973 9 Morgan, D. et al. (2000) A β peptide vaccination prevents memory loss in an animal model of Alzheimer’s disease. Nature 408, 982–985 10 Carlson, G.A. et al. (1997) Genetic modification of the phenotypes produced by amyloid precursor protein overexpression in transgenic mice. Hum. Mol. Genet. 6, 1951–1959 11 Sturchler-Pierrat, C. et al. (1997) Two amyloid precursor protein transgenic mouse models with Alzheimer disease-like pathology. Proc. Natl.Acad. Sci. U. S.A. 94, 13287–13292 12 Calhoun, M.E. et al. (1998) Neuron loss in APP transgenic mice. Nature 395, 755–756 13 Borchelt, D.R. et al. (1996) Familial Alzheimer’s disease-linked presenilin 1 variants elevate Aβ1-42/1-40 ratio in vitro and in vivo. Neuron 17, 1005–1013
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14 Borchelt, D.R. et al. (1997) Accelerated amyloid deposition in the brains of transgenic mice coexpressing mutant presenilin 1 and amyloid precursor proteins. Neuron 19, 939–945 15 Janus, C. et al. (2000) A β peptide immunization reduces behavioural impairment and plaques in a model of Alzheimer’s disease. Nature 408, 979–982 16 Chishti, M.A. et al. Early-onset amyloid deposition and cognitive deficits in transgenic mice expressing a double mutant form of APP695. J. Biol. Chem. (in press) 17 Moechars, D. et al. (1996) Expression in brain of amyloid precursor protein mutated in the α-secretase site causes disturbed behavior, neuronal degeneration and premature death in transgenic mice. EMBO J. 15, 1265–1274 18 Moechars, D. et al. (1998) Aggressive behaviour in transgenic mice expressing APP is alleviated by serotonergic drugs. NeuroReport 9, 3561–3564 19 Moechars, D. et al. (1999) Early phenotypic changes in transgenic mice that overexpress different mutants of amyloid precursor protein in brain. J. Biol. Chem. 274, 6483–6492 20 Moechars, D. et al. (1999) Premature death in transgenic mice that overexpress a mutant amyloid precursor protein is preceded by severe neurodegeneration and apoptosis. Neuroscience 91, 819–830 21 Reaume, A.G. et al. (1996) Enhanced amyloidogenic processing of the β-amyloid precursor protein in gene-targeted mice bearing the Swedish familial Alzheimer’s disease mutations and a ‘humanized’ Aβ sequence. J. Biol. Chem. 271, 23380–23388 22 Lamb, B.T. et al. (1993) Introduction and expression of the 400 kilobase amyloid precursor protein gene in transgenic mice. Nat.Genet. 5, 22–30
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23 Lamb, B.T. et al. (1997) Altered metabolism of familial Alzheimer’s disease-linked precursor protein variants in yeast artificial chromosome transgenic mice. Hum. Mol. Genet. 6, 1535–1541 24 Lamb, B.T. et al. (1999) Amyloid production and deposition in mutant amyloid precursor protein and presenilin-1 yeast artificial chromosome transgenic mice. Nat. Neurosci. 2, 695–697 25 Duff, K. et al. (1996) Increased amyloid-β42(43) in brains of mice expressing mutant presenilin 1. Nature 383, 710–713 26 Dewachter, I. et al. (2000) Aging increased amyloid peptide and caused amyloid plaques in brain of old APP/V717I transgenic mice by a different mechanism than mutant presenilin1. J. Neurosci. 20, 6452–6458 27 Siman, R. et al. (2000) Presenilin-1 P264L knock-in mutation: differential effects on Aβ production, amyloid deposition, and neuronal vulnerability. J. Neurosci. 20, 8717–8726 28 Irizarry, M.C. et al. (2000) Apolipoprotein E affects the amount, form, and anatomical distribution of amyloid β-peptide deposition in homozygous APP(V717F) transgenic mice. Acta Neuropathol. (Berl.) 100, 451–458 29 Mucke, L. et al. (2000) Astroglial expression of human α(1)-antichymotrypsin enhances Alzheimer-like pathology in amyloid protein precursor transgenic mice. Am. J. Pathol. 157, 2003–2010 30 Nilsson, L.N. et al. (2001) α-1-antichymotrypsin promotes beta-sheet amyloid plaque deposition in a transgenic mouse model of Alzheimer’s disease. J. Neurosci. 21, 1444–1451 31 Wyss-Coray,T. et al. (1997) Amyloidogenic role of cytokine TGF-β1 in transgenic mice and in Alzheimer’s disease. Nature 389, 603–606
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Transgenic mouse models of Alzheimer’s disease Brian J. Hock, Jr and Bruce T. Lamb Recent advances in the understanding of the genetic basis of Alzheimer’s disease have enabled the production of transgenic mouse models of the disease. Utilizing both cDNA- and genomic-based approaches, these mouse models for Alzheimer’s disease have already provided valuable insights into the pathogenesis of the disease and potential therapeutic interventions. Alzheimer’s disease (AD), the most common dementing disorder of later life, is a major cause of disability and death in the elderly1. Current estimates indicate that up to 10% of the population over the age of 65, and 50% over the age of 80, are afflicted by AD. Tragically, the progression of the disease is lengthy and there is currently no effective treatment. As people born during the population boom of the late 1940s and 1950s reach their 60s and 70s, AD will become a health care problem of catastrophic proportions. The cardinal clinical feature of AD is memory impairment that significantly worsens upon disease progression. A typical neuropsychological profile of an individual with mild to moderate AD includes recent declarative (explicit) memory impairments, deficits in problem solving, judgement and abstract reasoning, reduction in verbal fluency, and, often, language deficits. The memory loss exhibited in AD is dependent on the hippocampal system [dentate gyrus, cornu ammonis (CA)1–CA3, and rhinal cortices]. As the disease progresses, global amnesia, dependent on other cortical areas, debilitates the individual. Although dementia is the defining clinical feature of AD, a definitive diagnosis requires post-mortem examination of brain tissue for the presence of distinctive AD histopathology, namely filamentous inclusions in neuronal cell bodies (neurofibrillary tangles) and processes (neurites), extracellular deposits of the β-amyloid (Aβ) protein in senile plaques (Fig. 1) and within the walls of leptomeningeal/cerebral vessels, and synaptic and neuronal cell loss. At present, the relationship between these neuropathological observations and the onset of dementia remains quite controversial. In large part, this reflects the variability in the presentation of AD in terms of the length, severity and age-related progression of the disease. Accumulating evidence suggests that the development and progression of AD is subject to a wide variety of both environmental and genetic modifiers.
Genetics of Alzheimer’s disease Genetic investigations have demonstrated that AD is a heterogeneous disorder with several known etiologies: dosage imbalance for chromosome 21, as occurs in Down’s syndrome (DS); mutations in the gene encoding amyloid precursor protein (APP) on chromosome 21, the gene encoding presenilin-1 (PS1) on chromosome 14, and the gene encoding presenilin-2 (PS2) on chromosome 1, as occurs in autosomal dominant early-onset familial AD (FAD); and inheritance of distinct α-2 macroglobulin (A2M) and apolipoprotein E (ApoE) gene alleles on chromosomes 12 and 19, respectively, as significant genetic risk factors for late-onset FAD. Recently, numerous biochemical, biophysical and immunohistochemical studies have provided strong evidence that many of the genetic forms of AD share common pathogenic mechanisms that involve alterations in the metabolism of Aβ peptides. The predominant form of Aβ is a 39–43 amino acid peptide comprising 28 amino acids of the ectodomain and 11–15 amino acids of the adjacent transmembrane domain of APP (Fig. 2). The gene encoding APP encompasses 18 exons spanning 300 kb of DNA, giving rise to at least four tissue-specific alternatively spliced transcripts that encode proteins of 695, 714, 751 and 770 amino acids. APP, a type-I integral membrane glycoprotein of unknown function, matures through the constitutive secretory pathway. APP is preferentially processed by α-secretase cleavage, releasing the APP ectodomain (APPsα) into the extracellular space with retention of an intracellular C-terminal fragment (CTFα), which prevents the formation of full-length Aβ. Cleavage by β-secretase at the N-terminus of Aβ, however, releases a C-terminal APP fragment containing Aβ (CTFβ), which when followed by γ-secretase cleavage releases fulllength Aβ (Fig. 2) through pathways that remain obscure. Finally, APP can also be internalized and degraded through endosomal–lysosomal pathways.
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Brian J. Hock, Jr and Bruce T. Lamb* Dept of Genetics, Case Western Reserve University, University Hospitals of Cleveland, 10900 Euclid Avenue, Cleveland, OH 44106-4955, USA. *e-mail: [email protected]
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(a)
Figure 1. Deposition of Aβ in human Alzheimer’s disease and transgenic mouse brain Brain sections from (a) an end-stage sporadic human Alzheimer’s disease (AD) case and (b) a 14-month-old male TgR1.40 homozygous mutant APP YAC transgenic mouse, were stained with 6E10 — a human-specific monoclonal antibody for Aβ. Note the similarity of the Aβ deposits between human and mouse. Scale bars, 0.125 mm.
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(b)
Although the mechanisms and cell types involved in the generation, turnover and transport of Aβ and the formation of amyloid deposits in vivo remain poorly defined, several recent studies have provided insight into the secretases that cleave APP (Ref. 2). Two members of the disintegrin and metalloprotease family (ADAM), the membranebound tumor necrosis factor-α converting enzyme (TACE or ADAM17) and ADAM10, have been directly implicated in the α-secretase cleavage of APP. At present, however, it remains unclear which of these α-secretase activities contribute to the normal in vivo processing of APP and/or to the altered processing that results in AD. Recently, the enzymes responsible for β-secretase cleavage of APP were identified and termed the β-site APP cleaving enzyme (BACE) 1 and 2 (Ref. 2). BACE1 and BACE2 are transmembrane aspartyl proteases that can cleave both at the N-terminus of full-length Aβ and several locations within Aβ. Finally, although the exact identity of the γ-secretase remains a mystery, recent evidence suggests that PS1 normally indirectly or directly facilitates the proteolysis of APP by γ-secretase. The observation that older individuals with DS invariably develop AD histopathology suggests that dosage imbalance and the resulting overexpression of APP and Aβ leads to AD. In addition, in vitro transfection approaches, as well as studies using transgenic mice, have provided substantial insights into the mechanisms by which the FAD mutations affect the processing of APP and the production of Aβ (Fig. 2)1.Transfection studies have demonstrated that cells that express APP harboring the K670N/M671K or the D692G FAD substitutions, produce higher levels of CTFβ and secrete higher levels of all Aβ peptides as compared to cells expressing wildtype (wt) constructs through altered β-secretase cleavage. Cells that express APP harboring the I716V or the 717 (V717I, V717F, V717G and V717L) FAD substitutions do not secrete higher levels of Aβ, but rather release greater amounts of full-length Aβ peptides extending to residue 42 (Aβ1–42). Cells expressing the T714I mutant secrete higher levels of Aβ1–42, lower levels of Aβ1–40 and increased levels of N-terminally truncated Aβ ending at residue 42 (AβX–42), whereas the V715M mutation specifically increases the level of AβX–42. Taken together with immunocytochemical studies of human
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AD and DS cortex, as well as biophysical studies of Aβ, these results suggest that Aβ ending at residue 42 (either Aβ1–42 or AβX–42) is pathogenic, although the exact in vivo pathways that regulate these processes remain unclear. A majority of early-onset FAD pedigrees are linked to mutations in the PS1 gene on chromosome 14 and a much smaller percentage to mutations in the closely related PS2 gene on chromosome 1. The PS1/PS2 genes are highly conserved (67% identical in primary sequence) and both encompass 13 exons covering the ~70–100 kb that encode multiple alternatively spliced transcripts, which, in turn, give rise to polypeptides of 431–467 amino acids for PS1, and 415–448 amino acids for PS2. One hint about the pathological consequence of presenilin mutations comes from studies of Aβ metabolism, demonstrating an increased secretion of Aβ1–42 in cultured fibroblasts, plasma and brains from individuals with PS1 mutations, as well as transfected cell lines and transgenic mice.These studies strongly suggest that PS1 and PS2 variants could influence APP metabolism to promote the production of highly amyloidogenic Aβ1–42 peptides. Finally, current evidence suggests that both the late-onset FAD genes ApoE and A2M regulate the deposition and/or clearance of Aβ through pathways that remain obscure. Transgenic mouse models of Alzheimer’s disease One of the major difficulties in studying the relationship between the production and deposition of Aβ, the onset of dementia and the neuritic abnormalities, synaptic dysfunction and neuronal cell death that occurs in AD, has previously been the paucity of mouse models of the disease. Transgenic experiments can test directly whether the overexpression of wt or mutant APP, PS1 or PS2 genes leads to AD-type abnormalities and thus provide insight into the molecular mechanism(s) of disease. Over the past several years, numerous groups have generated mice with various features of AD using both cDNA- and genomicbased approaches, a number of which are summarized here with additional details provided in Table 1. cDNA-based transgenics Games et al.3 introduced a complex transgenic construct consisting of the platelet-derived growth factor (PDGF) β-chain promoter and a human APP mini-gene containing the APP Val717→Phe FAD mutation in an APP cDNA with portions of APP introns 6–8, enabling alternative splicing of exons 7 and 8. In the one transgenic line described (termed PD-APP) in a hybrid C57BL/6-DBA/2-Swiss Webster background, Southern blots disclosed ~40 copies of the transgene inserted at a single site and transmitted in a stable fashion. Levels of human APP mRNA were significantly greater than endogenous App transcripts; the three major splice variants of APP mRNA were demonstrable, albeit at dramatically abnormal ratios, and levels of APP were ~10-fold higher than endogenous mouse App. At
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eight months of age, Aβ deposits were observed in the hippocampus, corpus callosum and cerebral cortex; neurofibrillary tangles were not present (and not present in any of the other mouse models of AD). No significant neuronal loss was observed4, but it was recently reported that PD-APP mice were impaired in a novel spatial memory task in an age-dependent manner that correlated with the presence of Aβ deposits5. In a pioneering set of studies, immunization of either young or aged PD-APP mice with Aβ peptide resulted in a significant delay or reduction, respectively, of AD-like neuropathologies6. Hsiao et al.7 created transgenic mice (termed Tg2576) that express the 695 amino acid form of human APP containing the K670N/M671L FAD mutation, placed under the transcriptional control of the hamster prion gene promoter. In the single transgenic line described in a hybrid C57BL/6-SJL background, the levels of transgene-encoded APP were ~5.5-fold higher than endogenous mouse App, and at 9–11 months of age Aβ deposits were observed in numerous brain regions; neurofibrillary tangles were not present.These animals also developed behavioral disorders, including reduced spontaneous alternation performance in a Y-maze and increased escape latency in the water maze, although they failed to exhibit significant neuronal loss7,8. Recent data demonstrated that vaccination of Tg2576 mice with Aβ1–42 peptide reduces amyloid deposition and leaves spatial memory intact, compared with unvaccinated age-matched transgenics and controls9. Finally, the phenotypes observed in these mice are highly dependent on genetic background with embryonic lethality associated with backcrossing into certain inbred mouse strains10. Novartis Pharmaceuticals (Basel, Switzerland) developed two transgenic mouse models of AD expressing the 751 amino acid form of human APP in a hybrid C57BL/ 6-DBA/2 background11. In transgenic line TgAPP23, containing an APP cDNA with the K670N/M671L FAD mutation under the control of the murine Thy-1 promoter, the transgene was expressed sevenfold higher than endogenous App and the animals developed parenchymal and vascular Aβ deposits at six months of age. At 14 months of age, these mice displayed between 14 and 25% hippocampal CA1 neuronal loss that correlated with Aβ deposition12. Transgenic line TgAPP14/22, containing an APP cDNA with both the K670N/M671L and V717I mutations under the control of the human Thy-1 promoter, expressed the transgene twofold higher than endogenous App and developed Aβ deposits much later in life. Thus far, behavioral measurements have not been reported for these animals. Borchelt et al.13,14 developed an APP transgenic mouse (termed TgC3-3) using a mouse APP 695-cDNA with the L670N/M671L mutation and humanized Aβ (driven by the murine prion promoter) in a hybrid C3H/HeC57BL/6 background. These mice expressed the APP transgene at twofold higher levels than endogenous App, and developed Aβ deposits at 18 months of age. A similar cDNA transgenic model (termed TgCRND8) was recently
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Lipid bilayer
APP 751/770 KPI insert
Aβ
NH2 1
COOH 695 β-Secretase
α-Secretase
γ-Secretase (Aβ1– 42)
VKM DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA TVIVITLVMLK
K670N/M671L
A692G
T714I V715M I716V
L723P V717I V717G V717F V717L
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reported that contains an APP 695-cDNA with both the K670N/M671L and V717F mutations (under the control of the Syrian hamster prion promoter) in a hybrid C3H/He-C57BL/6 background15,16. The APP transgene was expressed at fivefold higher levels than endogenous APP, and Aβ deposits were observed as early as three months of age. It was also reported that immunization with Aβ1–42 reduces plaque load and behavioral impairments in TgCRND8 compared with nonimmunized Tg animals and controls. These mice also exhibit premature death that is dependent upon genetic background15. Van Leuven and colleagues17 generated a transgenic line (termed TgAPP/Ld/2) that contains the murine Thy-1 promoter driving expression of an APP-695 cDNA with the V717I mutation in the FVB/N background. The transgene was expressed at 25-fold higher levels than endogenous App mRNA, and fibrillar Aβ deposits develop from 13 months of age.These mice also show memory impairments in the water maze at three and six months of age. In addition, these mice exhibit premature death (72% at six months) and increased aggressiveness17–20. Genomic-based transgenics Although the cDNA-based transgenic mouse models of AD provided evidence that transgenic mice can develop Aβ deposits, it is unclear how the observed Aβ deposition relates to that observed in AD. Indeed, each of these cDNA-based transgenic studies relies on certain basic assumptions about the disease process with regard to the APP cDNA (695, 751 or 770 amino acid form) and the promoter (neuronal-specific, glial-specific, CNS-specific, brain region-specific, etc.) utilized to drive the temporal and spatial expression of the transgene. Because of these assumptions, it remains unclear how the formation of Aβ deposits in these transgenic mice will be related to the onset of behavioral impairments, the formation of neurofibrillary tangles, the neuronal cell loss and other phenotypes observed in individuals with AD. By contrast, the genomic-based approaches utilize the native human or mouse APP genomic sequence that contains the transcriptional regulatory elements required for proper spatial and
Figure 2. Schematic of the 695 isoform of amyloid precursor protein (APP) The figure depicts the site of the APP 751/770 Kunitz-type serine protease inhibitor (KPI) insert, highlighting an enlarged view of Aβ (purple box) and lipid bilayer (turquoise box) with the corresponding amino acid sequence. The α-, β- and γ-secretase cleavage sites are identified with arrows above the enlarged Aβ peptide with the various FAD mutations designated below. The FAD APP mutations are as follows: K670N/M671L is the Swedish mutation with the lysine→asparagine and methionine→leucine substitutions; A692G is the Flemish mutation (alanine→glycine); T714I is the Austrian mutation (threonine→isoleucine); V715M is the French mutation (valine→methionine); and I716V is the Florida mutation (isoleucine→valine); The 717 mutations are as follows: London, valine→isoleucine; valine→glycine; Indiana, valine→phenylalanine; and valine→leucine. L723P is the Australian mutation (leucine→proline). Transgenic mice with the Swedish, London or Indiana mutations are described in the review.
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Table 1. APP transgenic mouse models of Alzheimer’s diseasea Transgenic line
Mouse strainb
Approachc
Mutationd
Promoter e
APP expressionf
PD-APP
Swiss Webster, C57BL/6
Modified cDNA (695, 751, 770)
V717F
PDGF-β
10x
Tg2576
C57BL/6-SJL
cDNA (695)
Swedish
Hamster PrP
5–6x
Tg-APP23
C57BL/6-DBA/2, C57BL/6 cDNA (751)
Swedish
Murine Thy-1
7x
TgC3-3
C57BL/6-C3H
cDNA (695)
Swedish
Murine PrP
2x
TgCRND-8
C57BL/6-C3H
cDNA (695)
Swedish and V717I
Hamster PrP
5x
TgAPP/Ld/2
FVB/N
cDNA (695)
V717I
Murine Thy-1
25x (RNA)
TgAPP/Sw/1
FVB/N
cDNA (695)
Swedish
Murine Thy-1
7x (RNA)
TgAPPSwe-KI
CD-1
Genomic (695, 751, 770)
Swedish
Mouse App
0.5x
TgR1.40-YAC
C57BL/6, DBA/2,
Genomic (695, 751, 770)
Swedish
Human APP
Hemi/homo 2–3x/4–6x
129S3/SvIm, A aAbbreviations:
APP, amyloid precursor protein; n.r., not reported; PDGF, platelet-derived growth factor. strain of mouse or congenic generated. Designations separated by a hyphen represent hybrid genetic backgrounds. ccDNA or genomic-based [yeast artificial chromosome (YAC) or knock-in (KI)] transgenic approach, including the various APP cDNAs expressed from the construct (695, 751 or 770 amino acids). dAPP FAD mutation [V717F, V717I or Swedish (K670N/M671L)]. ePromoter utilized in transgenic construct. fLevels of APP expression (protein levels relative to endogenous mouse App unless otherwise specified). gAge of onset of Aβ deposits (earliest timepoint). hAcceleration of Aβ deposition of PS-1 FAD mutations. bInbred
temporal expression, with appropriate splice donor and acceptor sites needed to generate the entire spectrum of alternatively spliced transcripts (i.e. 695, 751 and 770). Thus, the genomic-based approach, unlike the cDNA-based transgenic approaches described above, make few assumptions about the molecular pathogenesis of AD, both in terms of the regional and temporal specificity of the disease. Utilizing a gene-targeting strategy, investigators at Cephalon (West Chester, PA, USA) humanized the mouse Aβ and introduced the K670N/M671L FAD mutation into a single transgenic line (termed TgAPPSwe-KI) in an outbred CD-1 background21. This unique approach resulted in the expression of human APP at 0.5-fold the level of endogenous App. Increased brain levels of human CTFβ and Aβ and decreased levels of APPsα were observed in a dosage-dependent manner. Neuropathology and behavior have not been described thus far. Another genomic-based approach relies on the introduction of the entire human AD genes cloned on yeast artificial chromosomes (YACs) into the germline of mice. Lamb et al.22,23 generated several wt and mutant APP YAC transgenic lines, including one (termed TgR1.40) with the entire 300 kb human APP gene harboring the K670N/M671L mutation on a hybrid C57BL/6-129/Sv background. Human-derived transgene products were expressed at two- to threefold the level of endogenous mouse App in the hemizygous state, and four- to sixfold in the homozygous state. Hemizygous and homozygous
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TgR1.40 animals develop Aβ deposits (primarily Aβ1–42) at 24–26 months and 14–16 months of age, respectively24. Recently, aged homozygous TgR1.40 mice were shown to exhibit behavioral impairments on spontaneous alternation, water maze retention and interference on the retention of an odor-guided task (Brian J. Hock, unpublished results).The TgR1.40 mice have recently been backcrossed into four genetic backgrounds including A, C57BL/6, DBA/2 and 129S3/SvIm (Bruce T. Lamb, unpublished results). Modifier genes The role of specific AD modifier genes has been tested in the APP transgenic mice described above by mating to other genetically modified mice. For example, several groups have demonstrated that transgene-derived expression of PS1 with FAD mutations results in elevation of Aβ1–42 and an earlier age of Aβ deposition in both cDNAand genomic-based APP transgenic mice14,24–27. Other genes that have been examined through crosses to APP transgenic mice include the different human ApoE isoforms28, α-1-antichymotrypsin29,30 and transforming growth factor (TGF) β131. Concluding remarks AD is a genetically heterogeneous disorder with many documented risk factors. After age, genetics is the second biggest risk factor. Utilizing information gleaned from
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Aβ deposits (age of onset)g
Acceleration by PS-1h
Neuritesi
NFT elementsj
Neuronal cell lossk
Behavioral impairmentsl
Immunizationm
Refs
6 months
n.r.
Yes
AT8 positive
Not detectable
Yes
Yes
3–6
9–12 months
Yes
Yes
AT8 positive
Not detectable
Yes
Yes
7–10
6 months
Yes
Yes
AT8 positive
25% in CA1 at
n.r.
n.r.
11,12
14 months 18 months
Yes
Yes
n.r.
n.r.
Yes
Yes
13,14
3–4 months
Yes
Yes
n.r.
Not detectable
Yes
Yes
15,16
13–18 months
Yes
Yes
AT8 positive
No overt loss
Yes
Yes
17–20
18 months
Yes
Yes
AT8 positive
No overt loss
n.r.
n.r.
19
n.r.
Yes
n.r.
n.r.
n.r.
n.r.
n.r.
21
Hemi/homo; 22 months/
Yes
Yes
AT8 positive
n.r.
Yes
n.r.
22–24
14 months iPresence
of neurites surrounding the Aβ deposits identified by silver staining, immunostaining and/or electron microscopy. Inflammation was identified in all mouse models by presence of microglia around the Aβ deposits and various inflammatory cytokines, as well as astrocytosis. jAlthough no neurofibrillary tangles (NFTs) have been observed in any APP transgenic mice, many exhibit the presence of hyperphosphorylated tau as documented by staining with phosphorylation-specific tau antibody AT8. kNeuronal cell loss as documented by cell counts in various brain regions. lBehavioral impairments observed in transgenic mice as documented by the Morris water maze (and modifications thereof), spatial alternation and other specific behavioral tasks. mImmunization performed with either Aβ peptide or anti-Aβ antibodies.
genetic studies of AD, various cDNA-based and genomicbased transgenic mouse models of AD have been developed. At present, the similarities and differences of the various mouse models remain unclear as they all have been developed with different APP cDNAs, FAD mutations, promoters and genetic background of the mouse strain. Future studies to examine the common and unique characteristics of these models in defined genetic backgrounds will be vital as they are currently being widely used by both academic and industry laboratories to examine the effects of therapeutic, environmental and genetic modification of phenotypes. Finally, although none of the transgenic models of AD recapitulate all of the features of the disease, these gene-based transgenic mouse models for AD have already provided valuable insights into the pathogenesis of the disease and potential therapeutic interventions that could prove efficacious in the treatment of AD. Acknowledgements We would like to thank M. Chiocco, E.H. Lehman and K. Pimpis for critical review of the manuscript. The Lamb laboratory is supported by NIH grant AG14451 and the Alzheimer’s Association grant IIRG-99-15157 (B.T.L.), by the NRSA Training Grant 5 T32 AG00105 and NIA 1 F32 AG05875-01A1 (B.J.H.) as well as support from the University Alzheimer Center (AG08012) and the Ireland Cancer Center (CA43703).
References 1 Selkoe, D.J. (2001) Alzheimer’s disease: genes, proteins, and therapy. Physiol. Rev. 81, 741–766 2 Nunan, J. and Small, D.H. (2000) Regulation of APP cleavage by α-, β- and γ-secretases. FEBS Lett. 483, 6–10 3 Games, D. et al. (1995) Alzheimer-type neuropathology in transgenic mice overexpressing V717F β-amyloid precursor protein. Nature 373, 523–527 4 Irizarry, M.C. et al. (1997) Aβ deposition is associated with neuropil change, but not overt neuronal loss in human amyloid precursor V717F (PD-APP) transgenic mice. J. Neurosci. 17, 7053–7059 5 Chen, G. et al. (2000) A learning deficit related to age and β-amyloid plaques in a mouse model of Alzheimer’s disease. Nature 408, 975–979 6 Schenk, D. et al. (1999) Immunization with amyloid-β attenuates Alzheimer-disease-like pathology in the PDAPP mouse. Nature 400, 173–177 7 Hsiao, K. et al. (1996) Correlative memory deficits, Aβ elevation and amyoid plaques in transgenic mice. Science 274, 99–102 8 Irizarry, M.C. et al. (1997) Tg (HuAPP695.K670N-M671L) 2576 mice develop age-related deposits and neuropil abnormalities, but no neuronal loss in CA1. J. Neuropathol. Exp. Neurol. 56, 965–973 9 Morgan, D. et al. (2000) A β peptide vaccination prevents memory loss in an animal model of Alzheimer’s disease. Nature 408, 982–985 10 Carlson, G.A. et al. (1997) Genetic modification of the phenotypes produced by amyloid precursor protein overexpression in transgenic mice. Hum. Mol. Genet. 6, 1951–1959 11 Sturchler-Pierrat, C. et al. (1997) Two amyloid precursor protein transgenic mouse models with Alzheimer disease-like pathology. Proc. Natl.Acad. Sci. U. S.A. 94, 13287–13292 12 Calhoun, M.E. et al. (1998) Neuron loss in APP transgenic mice. Nature 395, 755–756 13 Borchelt, D.R. et al. (1996) Familial Alzheimer’s disease-linked presenilin 1 variants elevate Aβ1-42/1-40 ratio in vitro and in vivo. Neuron 17, 1005–1013
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14 Borchelt, D.R. et al. (1997) Accelerated amyloid deposition in the brains of transgenic mice coexpressing mutant presenilin 1 and amyloid precursor proteins. Neuron 19, 939–945 15 Janus, C. et al. (2000) A β peptide immunization reduces behavioural impairment and plaques in a model of Alzheimer’s disease. Nature 408, 979–982 16 Chishti, M.A. et al. Early-onset amyloid deposition and cognitive deficits in transgenic mice expressing a double mutant form of APP695. J. Biol. Chem. (in press) 17 Moechars, D. et al. (1996) Expression in brain of amyloid precursor protein mutated in the α-secretase site causes disturbed behavior, neuronal degeneration and premature death in transgenic mice. EMBO J. 15, 1265–1274 18 Moechars, D. et al. (1998) Aggressive behaviour in transgenic mice expressing APP is alleviated by serotonergic drugs. NeuroReport 9, 3561–3564 19 Moechars, D. et al. (1999) Early phenotypic changes in transgenic mice that overexpress different mutants of amyloid precursor protein in brain. J. Biol. Chem. 274, 6483–6492 20 Moechars, D. et al. (1999) Premature death in transgenic mice that overexpress a mutant amyloid precursor protein is preceded by severe neurodegeneration and apoptosis. Neuroscience 91, 819–830 21 Reaume, A.G. et al. (1996) Enhanced amyloidogenic processing of the β-amyloid precursor protein in gene-targeted mice bearing the Swedish familial Alzheimer’s disease mutations and a ‘humanized’ Aβ sequence. J. Biol. Chem. 271, 23380–23388 22 Lamb, B.T. et al. (1993) Introduction and expression of the 400 kilobase amyloid precursor protein gene in transgenic mice. Nat.Genet. 5, 22–30
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23 Lamb, B.T. et al. (1997) Altered metabolism of familial Alzheimer’s disease-linked precursor protein variants in yeast artificial chromosome transgenic mice. Hum. Mol. Genet. 6, 1535–1541 24 Lamb, B.T. et al. (1999) Amyloid production and deposition in mutant amyloid precursor protein and presenilin-1 yeast artificial chromosome transgenic mice. Nat. Neurosci. 2, 695–697 25 Duff, K. et al. (1996) Increased amyloid-β42(43) in brains of mice expressing mutant presenilin 1. Nature 383, 710–713 26 Dewachter, I. et al. (2000) Aging increased amyloid peptide and caused amyloid plaques in brain of old APP/V717I transgenic mice by a different mechanism than mutant presenilin1. J. Neurosci. 20, 6452–6458 27 Siman, R. et al. (2000) Presenilin-1 P264L knock-in mutation: differential effects on Aβ production, amyloid deposition, and neuronal vulnerability. J. Neurosci. 20, 8717–8726 28 Irizarry, M.C. et al. (2000) Apolipoprotein E affects the amount, form, and anatomical distribution of amyloid β-peptide deposition in homozygous APP(V717F) transgenic mice. Acta Neuropathol. (Berl.) 100, 451–458 29 Mucke, L. et al. (2000) Astroglial expression of human α(1)-antichymotrypsin enhances Alzheimer-like pathology in amyloid protein precursor transgenic mice. Am. J. Pathol. 157, 2003–2010 30 Nilsson, L.N. et al. (2001) α-1-antichymotrypsin promotes beta-sheet amyloid plaque deposition in a transgenic mouse model of Alzheimer’s disease. J. Neurosci. 21, 1444–1451 31 Wyss-Coray,T. et al. (1997) Amyloidogenic role of cytokine TGF-β1 in transgenic mice and in Alzheimer’s disease. Nature 389, 603–606