It is becoming increasingly evident that different flavors of APP/Aβ may modulate different functions. This paper contributes additional information to this topic.

View all comments by Paul Coleman

APP Cytoplasmic Domain: An Orphan No More!
A central tenet of the “amyloid cascade hypothesis” posits that Aβ peptides are the causative agent of AD pathogenesis. Although the details remain sketchy, the amyloid hypothesis suggests that abnormal accumulation of Aβ peptides triggers a cascade of events that cause synaptic loss and cell death, resulting ultimately in AD. Indeed, genetic data strongly implicate Aβ in AD as the FAD mutations in APP mostly flank the Aβ region and a majority of FAD mutations are found in the presenilin gene, which encodes the key component of the γ-secretase complex. Although the initial notion that the senile plaques—the accumulated stores of Aβ peptides—cause AD has now gone out of favor, a current view favors Aβ oligomers to be the real culprit. The wealth of literature implicating Aβ in AD notwithstanding, it is clear that we do not know how or which form of Aβ causes AD and whether Aβ alone can account for all facets of AD pathogenesis. This paper by Galvan et al. (1) in the May 2 issue of the PNAS adds an interesting twist to this...  Read more

APP Cytoplasmic Domain: An Orphan No More!
A central tenet of the “amyloid cascade hypothesis” posits that Aβ peptides are the causative agent of AD pathogenesis. Although the details remain sketchy, the amyloid hypothesis suggests that abnormal accumulation of Aβ peptides triggers a cascade of events that cause synaptic loss and cell death, resulting ultimately in AD. Indeed, genetic data strongly implicate Aβ in AD as the FAD mutations in APP mostly flank the Aβ region and a majority of FAD mutations are found in the presenilin gene, which encodes the key component of the γ-secretase complex. Although the initial notion that the senile plaques—the accumulated stores of Aβ peptides—cause AD has now gone out of favor, a current view favors Aβ oligomers to be the real culprit. The wealth of literature implicating Aβ in AD notwithstanding, it is clear that we do not know how or which form of Aβ causes AD and whether Aβ alone can account for all facets of AD pathogenesis. This paper by Galvan et al. (1) in the May 2 issue of the PNAS adds an interesting twist to this story and reminds us to keep an open mind when thinking of AD pathogenesis.

Galvan et al. present an intriguing observation that introduction of a point mutation in the cytoplasmic domain of a FAD mutant APP prevents the AD-like pathology and behavioral deficits without a reduction in Aβ levels or plaque deposition. Dale Bredesen, the senior author of the study, and Eddie Koo had previously reported (2) that the consensus caspase cleavage site (661VEVD664, numbering according to APP695) in the APP cytoplasmic domain may be cleaved when the apoptotic program is activated (also independently shown by Gervais et al. [3]). Koo and Bredesen also demonstrated that C-terminal 31-residue-long peptide comprising APP 665-695 (termed C31), a product of such a cleavage event, was proapoptotic and put forward a model (4) based on cell culture data in which Aβ binds and dimerizes APP, triggering the cleavage of the APP at the VEVD motif and releasing the cytotoxic C31 fragment. Interestingly, the work of Bernadette Allinquant (5) suggests that the upstream peptide comprising APP 649-664 (termed Jcasp), also a product of cleavage at D664, activates caspases and induces neuronal death.

To determine the contribution of this cleavage event to AD pathogenesis, the Bredesen group made use of the PDAPP mouse model of AD. These mice express human APP bearing the Swedish and Indiana mutations, produce higher amounts of Aβ40 and Aβ42, and develop Aβ plaques by 12-15 months of age. These mice also show synaptic loss, astrogliosis, dentate gyral atrophy, and behavioral deficits in Morris water maze and Y-maze tests. Galvan et al., introduced a mutation changing Asp664 to Ala in the APP cytoplasmic domain, which is presumably resistant to cleavage by caspases. The authors report that their “triple”-transgenic mice (bearing the Swedish, Indiana, and D664A mutations) produced comparable amounts of Aβ and formed similar plaque load when compared to PDAPP mice. Unexpectedly, introduction of the D664A mutation completely prevented synaptic loss, astrogliosis, and memory deficits seen in PDAPP mice. The authors conclude that the APP cytoplasmic domain plays a key role in the development of AD-like deficits in transgenic mice. The authors claim that this observation represents a prevention or reversal of AD-like pathology and behavioral deficits. However, keeping in mind that these are independently generated lines with different integration sites, it is perhaps more appropriate to conclude that these mice fail to show the expected deficits. It should be noted that the authors do present data from two independent lines, and both show a lack of behavioral deficits.

Since we do not know how D664A mutation counters the AD-like deficits in PDAPP mice, the findings of Galvan et al. can be interpreted in many ways. Some might conclude these data to show that Aβ plays no or only a minor role in the development of AD pathogenesis. However, others may point to the wealth of literature implicating Aβ in AD and dismiss such a drastic conclusion. The authors seem to favor the view that the cleavage of the APP cytoplasmic domain at D664 lies downstream from events initiated by Aβ. Another possibility is that D664A mutation alters the binding of the APP cytoplasmic domain with its interacting partners and thereby prevents the deleterious effects observed in PDAPP mice. One issue the authors did not consider is what happens to APP intracellular domain (AICD) produced from D664A bearing APP in their transgenic mice. There is increasing awareness that AICD could play a significant role in APP function and/or AD pathogenesis (6). Since D664A mutation results in the loss of a negative charge and takes place near the T668 residue (phosphorylation of which alters binding with Fe65), it is not unlikely that D664A mutation results in major alterations in protein-protein interaction involving both the APP cytoplasmic domain and AICD.

The effects of D664A mutation in preventing AD-like deficits are so striking that these studies are likely to be repeated in other mouse models of AD. Although the cellular basis of the effects of D664A mutation in blocking the synaptic loss and behavioral deficits remains obscure, these findings demonstrate a key role for APP cytoplasmic domain (and/or AICD) in AD pathogenesis in the PDAPP mouse model. Irrespective of whether it plays a role downstream of Aβ or independent of Aβ, these findings further add to the increasing importance of APP cytoplasmic domain/AICD in APP pathophysiology. This paper also shows that a more open view of AD pathogenesis will be a welcome change.

References:
1. Galvan V, Gorostiza OF, Banwait S, Ataie M, Logvinova AV, Sitaraman S, Carlson E, Sagi SA, Chevallier N, Jin K, Greenberg DA, Bredesen DE. Reversal of Alzheimer's-like pathology and behavior in human APP transgenic mice by mutation of Asp664. Proc Natl Acad Sci U S A. 2006 May 2;103(18):7130-5. Epub 2006 Apr 25. Abstract

2. Lu DC, Rabizadeh S, Chandra S, Shayya RF, Ellerby LM, Ye X, Salvesen GS, Koo EH, Bredesen DE. A second cytotoxic proteolytic peptide derived from amyloid beta-protein precursor. Nat Med. 2000 Apr;6(4):397-404. Abstract

3. Gervais FG, Xu D, Robertson GS, Vaillancourt JP, Zhu Y, Huang J, LeBlanc A, Smith D, Rigby M, Shearman MS, Clarke EE, Zheng H, Van Der Ploeg LH, Ruffolo SC, Thornberry NA, Xanthoudakis S, Zamboni RJ, Roy S, Nicholson DW. Involvement of caspases in proteolytic cleavage of Alzheimer's amyloid-beta precursor protein and amyloidogenic A beta peptide formation. Cell. 1999 Apr 30;97(3):395-406. Abstract

4. Lu DC, Shaked GM, Masliah E, Bredesen DE, Koo EH. Amyloid beta protein toxicity mediated by the formation of amyloid-beta protein precursor complexes. Ann Neurol. 2003 Dec;54(6):781-9. Abstract

5. Madeira A, Pommet JM, Prochiantz A, Allinquant B. SET protein (TAF1beta, I2PP2A) is involved in neuronal apoptosis induced by an amyloid precursor protein cytoplasmic subdomain. FASEB J. 2005 Nov;19(13):1905-7. Epub 2005 Sep 14. Abstract

6. Ryan KA, Pimplikar SW. Activation of GSK-3 and phosphorylation of CRMP2 in transgenic mice expressing APP intracellular domain. J Cell Biol. 2005 Oct 24;171(2):327-35. Epub 2005 Oct 17. Abstract

View all comments by Sanjay W. Pimplikar

I have read this article with great interest since I believe it is always refreshing to hear about alternative views that explain the deficits characteristic of Alzheimer disease. This paper provides evidence that, whereas amyloid production and plaque formation were unaltered, synaptic loss and behavioral abnormalities were completely prevented by mutation at a functional Asp664 caspase cleavage site. This site was described in a previously published paper from Konrad Beyreuther’s laboratory in Heidelberg (see Weidemann et al., 1999) and was suggested to regulate programmed cell death. Meanwhile, we know that this site is within the APP intracellular domain, which has been named AICD and which consists of the last 50 carboxy-terminal residues of the APP protein.

The AICD fragment, like NICD, can complex with transcription factors. Unfortunately, since the authors did not pay attention to the AICD molecule, we do not know what effect the D664A mutation has on the AICD production, stability and its transport to the nucleus. While...  Read more

I have read this article with great interest since I believe it is always refreshing to hear about alternative views that explain the deficits characteristic of Alzheimer disease. This paper provides evidence that, whereas amyloid production and plaque formation were unaltered, synaptic loss and behavioral abnormalities were completely prevented by mutation at a functional Asp664 caspase cleavage site. This site was described in a previously published paper from Konrad Beyreuther’s laboratory in Heidelberg (see Weidemann et al., 1999) and was suggested to regulate programmed cell death. Meanwhile, we know that this site is within the APP intracellular domain, which has been named AICD and which consists of the last 50 carboxy-terminal residues of the APP protein.

The AICD fragment, like NICD, can complex with transcription factors. Unfortunately, since the authors did not pay attention to the AICD molecule, we do not know what effect the D664A mutation has on the AICD production, stability and its transport to the nucleus. While the authors claim in their paper that Asp664 does not affect Aβ production but is critical for synaptic loss, this conclusion is based on analysis of soluble Aβ by ELISA that detects Aβ40 and Aβ42, and by Western analysis. Here a biochemist would like to see a more detailed analysis by MS, etc. It would also be of interest to see how intracellular Aβ is affected since intraneuronal Aβ accumulation precedes plaque formation in APP and PS1 double-transgenic mice (see Wirths et al., 2001). At the end, it may turn out that the authors are right to believe that the cleavage of APP at Asp664 represents a therapeutic target but possibly is also due to effects on the consecutive cleavage mechanism of γ-secretase (see Qi-Takahara et al., 2005) or intracellular Aβ species, or both.

View all comments by Gerd Multhaup

Amyloid-β: The Finger or the Moon?
The study by Galvan et al. (Galvan et al., 2006) provides another clear example that amyloid-β is not responsible for the cognitive and pathological changes that stereotypify Alzheimer disease (AD) (Lee et al., 2004; Lee et al., 2006). Specifically, the researchers demonstrate that by introducing an additional mutation to PDAPP mice that prevents the cleavage of APP by caspase, while not affecting amyloid-β, rescues cognitive and pathological deficits in PDAPP transgenic mice. Taken together with recent findings that knockout of presenilin 1 (i.e., no amyloid-β), while abrogating amyloid-β pathology from APP mutant transgenic mice, failed to rescue cognitive deficits (Dewachter et al., 2002; Saura et al., 2005), there can only be one conclusion: Amyloid is not responsible for cognitive deficits. Indeed, this conclusion is based on both negative and positive correlates (i.e., cognitive deficits with no amyloid-β and rescue of cognitive deficits without change of amyloid-β). It is surprising that the field still...  Read more

Amyloid-β: The Finger or the Moon?
The study by Galvan et al. (Galvan et al., 2006) provides another clear example that amyloid-β is not responsible for the cognitive and pathological changes that stereotypify Alzheimer disease (AD) (Lee et al., 2004; Lee et al., 2006). Specifically, the researchers demonstrate that by introducing an additional mutation to PDAPP mice that prevents the cleavage of APP by caspase, while not affecting amyloid-β, rescues cognitive and pathological deficits in PDAPP transgenic mice. Taken together with recent findings that knockout of presenilin 1 (i.e., no amyloid-β), while abrogating amyloid-β pathology from APP mutant transgenic mice, failed to rescue cognitive deficits (Dewachter et al., 2002; Saura et al., 2005), there can only be one conclusion: Amyloid is not responsible for cognitive deficits. Indeed, this conclusion is based on both negative and positive correlates (i.e., cognitive deficits with no amyloid-β and rescue of cognitive deficits without change of amyloid-β). It is surprising that the field still remains convinced that amyloid is important.

“When a sage points to the moon, some people see the finger and not the moon.”

References:
Dewachter I, Reverse D, Caluwaerts N, Ris L, Kuiperi C, Van den Haute C, Spittaels K, Umans L, Serneels L, Thiry E, Moechars D, Mercken M, Godaux E, Van Leuven F. Neuronal deficiency of presenilin 1 inhibits amyloid plaque formation and corrects hippocampal long-term potentiation but not a cognitive defect of amyloid precursor protein [V717I] transgenic mice. J Neurosci. 2002 May 1;22(9):3445-53. Abstract

Galvan V, Gorostiza OF, Banwait S, Ataie M, Logvinova AV, Sitaraman S, Carlson E, Sagi SA, Chevallier N, Jin K, Greenberg DA, Bredesen DE. Reversal of Alzheimer's-like pathology and behavior in human APP transgenic mice by mutation of Asp664. Proc Natl Acad Sci U S A. 2006 May 2;103(18):7130-5. Epub 2006 Apr 25. Abstract

Lee HG, Casadesus G, Zhu X, Takeda A, Perry G, Smith MA. Challenging the amyloid cascade hypothesis: senile plaques and amyloid-beta as protective adaptations to Alzheimer disease. Ann N Y Acad Sci. 2004 Jun;1019:1-4. Review. Abstract

Lee HG, Zhu X, Nunomura A, Perry G, Smith MA. Amyloid beta: the alternate hypothesis. Curr Alzheimer Res. 2006 Feb;3(1):75-80. Review. Abstract

Saura CA, Chen G, Malkani S, Choi SY, Takahashi RH, Zhang D, Gouras GK, Kirkwood A, Morris RG, Shen J. Conditional inactivation of presenilin 1 prevents amyloid accumulation and temporarily rescues contextual and spatial working memory impairments in amyloid precursor protein transgenic mice. J Neurosci. 2005 Jul 20;25(29):6755-64. Abstract

View all comments by Hyoung-gon Lee
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View all comments by Xiongwei Zhu

This paper, again, indicates that Abeta is not the sole factor for AD development. It will be very exciting to see how all the contributing factors (e.g oxidative stress, protein synthesis/degradation, calcium etc) interact in the development of AD.

View all comments by H. Fai Poon

The study by Galvan et al. is the first to show the involvement of APP cytoplasmic caspase cleavage in the development of AD pathology. The authors substituted the aspartate at position 664 of APP with an alanine in a minigene carrying familial AD mutated human APP downstream from the PDGF B chain promoter. From this they generated mice in which Aβ production and deposits are unaltered, in contrast to dentate gyral atrophy, synaptic loss, astrogliosis, increased proliferation of dentate gyrus cells, and associated cognitive abnormalities, which were all attenuated. These data show that caspase cleavage at Asp664 plays a critical role in the development of AD-like deficits in the mouse model.

This caspase cleavage can be induced by soluble amyloid peptide, as previously reported (Lu et al., 2003). Interestingly, the absence of cleavage site does not modify Aβ formation. Caspase cleavage alone, or in association with γ-secretase cleavage, can generate toxic peptides as previously observed in vitro. As suggested by the authors, besides the formation of these toxic peptides,...  Read more

The study by Galvan et al. is the first to show the involvement of APP cytoplasmic caspase cleavage in the development of AD pathology. The authors substituted the aspartate at position 664 of APP with an alanine in a minigene carrying familial AD mutated human APP downstream from the PDGF B chain promoter. From this they generated mice in which Aβ production and deposits are unaltered, in contrast to dentate gyral atrophy, synaptic loss, astrogliosis, increased proliferation of dentate gyrus cells, and associated cognitive abnormalities, which were all attenuated. These data show that caspase cleavage at Asp664 plays a critical role in the development of AD-like deficits in the mouse model.

This caspase cleavage can be induced by soluble amyloid peptide, as previously reported (Lu et al., 2003). Interestingly, the absence of cleavage site does not modify Aβ formation. Caspase cleavage alone, or in association with γ-secretase cleavage, can generate toxic peptides as previously observed in vitro. As suggested by the authors, besides the formation of these toxic peptides, cleavage at Asp664 could also prevent some conformational structure necessary for interactions required for APP trafficking and function, which remain to be determined. The mechanisms responsible for the AD-like deficits observed in the transgenic mouse model require further investigation.

Although the involvement of caspases in AD pathology is still controversial, several reports have described the presence of activated caspases in AD brains, and of caspase-cleaved APP, suggesting that apoptosis and related protein caspase cleavage, including that of APP, may have a role in AD pathology (Lu et al., 2000; Lu et al., 2003; Zhao et al., 2003; Guo et al., 2004). Taken together, this study, in an AD transgenic mouse model, confirms that APP caspase cleavage plays an important role in AD pathology, and may be a new therapeutic target to investigate.

References:
Lu DC, Rabizadeh S, Chandra S, Shayya RF, Ellerby LM, Salvesen GS, Koo EH, Bredesen DE. A second cytotoxic proteolytic peptide derived from amyloid β-protein precursor. Nat Med. 2000 Apr;6(4):385-6. Abstract

Lu DC, Shaked GM, Masliah E, Bredesen DE, Koo EH. Amyloid β protein toxicity mediated by the formation of amyloid-β protein precursor complexes. Ann Neurol. 2003 Dec;54(6):781-9. Abstract

Guo H, Albrecht S, Bourdeau M, Petzke T, Bergeron C, LeBlanc A. Active caspase-6 and caspase-6-cleaved tau in neuropil threads, neuritic plaques, and neurofibrillar tangles of Alzheimer’s disease. Am J Pathol. 2004 Aug;165(2):523-31. Abstract

Zhao M, Su J, Head E, Cotman CW. Accumulation of caspase cleaved amyloid precursor protein represents an early neurodegenerative event in aging and in Alzheimer's disease. Neurobiol Dis. 2003 Dec;14(3):391-403. Abstract

View all comments by Bernadette ALLINQUANT