β - but not γ-secretase proteolysis of APP causes synaptic and memory deficits in a mouse model of dementia

Department of Microbiology and Immunology, Albert Einstein College of Medicine, Bronx, NY, USA.
EMBO Molecular Medicine (Impact Factor: 8.25). 03/2012; 4(3):171-9. DOI: 10.1002/emmm.201100195
Source: PubMed

ABSTRACT A mutation in the BRI2/ITM2b gene causes loss of BRI2 protein leading to familial Danish dementia (FDD). BRI2 deficiency of FDD provokes an increase in amyloid-β precursor protein (APP) processing since BRI2 is an inhibitor of APP proteolysis, and APP mediates the synaptic/memory deficits in FDD. APP processing is linked to Alzheimer disease (AD) pathogenesis, which is consistent with a common mechanism involving toxic APP metabolites in both dementias. We show that inhibition of APP cleavage by β-secretase rescues synaptic/memory deficits in a mouse model of FDD. β-cleavage of APP yields amino-terminal-soluble APPβ (sAPPβ) and β-carboxyl-terminal fragments (β-CTF). Processing of β-CTF by γ-secretase releases amyloid-β (Aβ), which is assumed to cause AD. However, inhibition of γ-secretase did not ameliorate synaptic/memory deficits of FDD mice. These results suggest that sAPPβ and/or β-CTF, rather than Aβ, are the toxic species causing dementia, and indicate that reducing β-cleavage of APP is an appropriate therapeutic approach to treating human dementias. Our data and the failures of anti-Aβ therapies in humans advise against targeting γ-secretase cleavage of APP and/or Aβ.

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Available from: Luciano D'adamio, Aug 22, 2015
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    • "(See legend on next page.) Tamayev et al. Molecular Neurodegeneration 2012, 7:60 Page 6 of 11 disulfide linkage. This releases the peptide cargo and allows it to act at its target. We have previously shown that Pen1-XBIR3 inhibits caspase-9 dependent cell death using primary hippocampal neuron cultures, and that Pen1-XBIR3 delivery to the CNS blocks casp"
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    ABSTRACT: Background Mutations in either Aβ Precursor protein (APP) or genes that regulate APP processing, such as BRI2/ITM2B and PSEN1/PSEN2, cause familial dementias. Although dementias due to APP/PSEN1/PSEN2 mutations are classified as familial Alzheimer disease (FAD) and those due to mutations in BRI2/ITM2B as British and Danish dementias (FBD, FDD), data suggest that these diseases have a common pathogenesis involving toxic APP metabolites. It was previously shown that FAD mutations in APP and PSENs promote activation of caspases leading to the hypothesis that aberrant caspase activation could participate in AD pathogenesis. Results Here, we tested whether a similar mechanism applies to the Danish BRI2/ITM2B mutation. We have generated a genetically congruous mouse model of FDD, called FDDKI, which presents memory and synaptic plasticity deficits. We found that caspase-9 is activated in hippocampal synaptic fractions of FDDKI mice and inhibition of caspase-9 activity rescues both synaptic plasticity and memory deficits. Conclusion These data directly implicate caspase-9 in the pathogenesis of Danish dementia and suggest that reducing caspase-9 activity is a valid therapeutic approach to treating human dementias.
    Molecular Neurodegeneration 12/2012; 7(1):60. DOI:10.1186/1750-1326-7-60 · 5.29 Impact Factor
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    • "Therefore, it is possible that APP processing is altered less dramatically in FBD, and that this reduction cannot be detected when analyzing the steady-state levels on endogenous APP metabolites in the CNS. The recent finding that memory deficits of FDD KI mice can be rescued inhibiting β-secretase cleavage of APP (R. Tamayev et al., 2011a) supports this hypothesis. Alternatively, it could be postulated that APP, BRI2 and APP/BRI2 complexes deliver distinct signals. "
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    ABSTRACT: Familial British Dementia (FBD) is caused by an autosomal dominant mutation in the BRI2/ITM2B gene (Vidal et al., 1999). FBD(KI) mice are a model of FBD that is genetically congruous to the human disease, because they carry one mutant and one wild-type Bri2/Itm2b allele. Analysis of these mice has shown that the British mutation causes memory impairments due to loss of Bri2 function (Tamayev et al., 2010b). BRI2 is a physiologic inhibitor of processing of the Aβ-precursor protein (APP; Matsuda et al., 2008), a gene associated with Alzheimer's disease (Bertram et al., 2010). Here we show that APP haploinsufficiency prevents memory dysfunctions seen in FBD(KI) mice. This genetic suppression is consistent with a role for APP in the pathogenesis of memory deficits. Moreover, it provides compelling evidence that the memory dysfunctions caused by the British BRI2 mutant are dependent on endogenous APP and that BRI2 and APP functionally interact. This evidence establishes a mechanistic connection between Familial British and Alzheimer's dementias.
    The Journal of Neuroscience : The Official Journal of the Society for Neuroscience 04/2012; 32(16):5481-5. DOI:10.1523/JNEUROSCI.5193-11.2012 · 6.75 Impact Factor
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    ABSTRACT: A mutation in the BRI2/ITM2b gene causes familial Danish dementia (FDD). BRI2 is an inhibitor of amyloid-β precursor protein (APP) processing, which is genetically linked to Alzheimer's disease (AD) pathogenesis. The FDD mutation leads to a loss of BRI2 protein and to increased APP processing. APP haplodeficiency and inhibition of APP cleavage by β-secretase rescue synaptic/memory deficits of a genetically congruous mouse model of FDD (FDDKI). β-cleavage of APP yields the β-carboxyl-terminal (β-CTF) and the amino-terminal-soluble APPβ (sAPPβ) fragments. γ-secretase processing of β-CTF generates Aβ, which is considered the main cause of AD. However, inhibiting Aβ production did not rescue the deficits of FDDKI mice, suggesting that sAPPβ/β-CTF, and not Aβ, are the toxic species causing memory loss. Here, we have further analyzed the effect of γ-secretase inhibition. We show that treatment with a γ-secretase inhibitor (GSI) results in a worsening of the memory deficits of FDDKI mice. This deleterious effect on memory correlates with increased levels of the β/α-CTFs APP fragments in synaptic fractions isolated from hippocampi of FDDKI mice, which is consistent with inhibition of γ-secretase activity. This harmful effect of the GSI is in sharp contrast with a pathogenic role for Aβ, and suggests that the worsening of memory deficits may be due to accumulation of synaptic-toxic β/α-CTFs caused by GSI treatment. However, γ-secretase cleaves more than 40 proteins; thus, the noxious effect of GSI on memory may be dependent on inhibition of cleavage of one or more of these other γ-secretase substrates. These two possibilities do not need to be mutually exclusive. Our results are consistent with the outcome of a clinical trial with the GSI Semagacestat, which caused a worsening of cognition, and advise against targeting γ-secretase in the therapy of AD. Overall, the data also indicate that FDDKI is a valuable mouse model to study AD pathogenesis and predict the clinical outcome of therapeutic agents for AD.
    Molecular Neurodegeneration 04/2012; 7(1):19. DOI:10.1186/1750-1326-7-19 · 5.29 Impact Factor
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