The experimental design is very elegant and leaves no doubt that...  Read more

The experimental design is very elegant and leaves no doubt that γ-secretase function is essential for neuronal survival. Although the exact mechanism that connects γ-secretase activity with neuronal survival is not established, these findings have obvious and important implications for developing and clinically testing γ-secretase inhibitors. The conclusion about the non-essential role of ER Ca2+ leak function of presenilins is somewhat less strong. The authors assume that ER Ca2+ signaling is normal in adult neurons from nicastrin cKO mice, but did not directly test it in the paper. In the previous studies our laboratory demonstrated that ER Ca2+ signals are normal in Aph TKO MEF cells (2), so it is already known that inactivation of γ-secretase per se does not have a major effect on ER Ca2+ handling. However, it is possible that levels of presenilin expression in aging neurons from nicastrin cKO mice are sufficiently reduced to affect ER Ca2+ signaling. When cortical lysates from two-month-old mice were analyzed by Western blotting, levels of presenilin-1 were reduced approximately twofold in nicastrin cKO mice. Unfortunately, the expression levels of presenilins are not shown for six- and nine-month cortical lysates from nicastrin cKO mice.

Another critical issue to consider is whether memory deficits and cortical neuronal loss observed in PS cDKO and nicastrin cKO mice faithfully replicate neuronal dysfunction and eventual death in AD. There is no doubt that previous (1) and present (Tabuchi at al., 2009) data from Jie Shen’s lab indicate that γ-secretase activity is important for neuronal survival. However, it remains unclear if the pathway leading to neuronal cell death in γ-secretase KO models is the same as in AD. This is a critical question that still needs to be addressed. Accumulation of amyloid has been postulated to play a critical role in AD pathogenesis (3). Abnormal neuronal Ca2+ signaling has also been implicated as one of the pathogenic pathways involved in AD (4). PS cDKO and nicastrin cKO mice do not produce amyloid and nicastrin cKO mice presumably have normal Ca2+ signaling. Thus, an open question is if neuronal cell death observed in these γ-secretase knockout mice is a faithful model for neuronal cell death in AD. In any case, the new results obtained by the authors provide very interesting and important insights into a connection between γ-secretase activity and neuronal survival. Understanding the mechanistic basis responsible for this connection will be an extremely important future task.

References:
1. Saura, C.A. et al. (2004) Loss of presenilin function causes impairments of memory and synaptic plasticity followed by age-dependent neurodegeneration. Neuron 42, 23-36. Abstract

2. Tu, H. et al. (2006) Presenilins form ER calcium leak channels, a function disrupted by mutations linked to familial Alzheimer's disease. Cell 126, 981-993. Abstract

3. Hardy, J. and Selkoe, D.J. (2002) The amyloid hypothesis of Alzheimer's disease: progress and problems on the road to therapeutics. Science 297, 353-6. Abstract

4. Bezprozvanny, I. and Mattson, M.P. (2008) Neuronal calcium mishandling and the pathogenesis of Alzheimer's disease. Trends Neurosci 31, 454-63. Abstract

View all comments by Ilya Bezprozvanny

The challenge will be now to determine what are the signaling pathways—downstream from γ-secretase—involved in neuronal death in the aging brain. Cadherins may represent attractive candidates. Indeed, the cadherin family of cell-cell adhesion proteins is abundantly expressed at mature synapses, is critical for synaptic plasticity, and is cleaved by γ-secretase in neurons upon NMDA receptor stimulation (Marambaud et al., 2003). It is, therefore, reasonable to think that a loss of synaptic cadherin cleavage by γ-secretase may lead over time to defects in synaptic plasticity and neuronal integrity and thus may contribute to the phenotype observed in...  Read more

The challenge will be now to determine what are the signaling pathways—downstream from γ-secretase—involved in neuronal death in the aging brain. Cadherins may represent attractive candidates. Indeed, the cadherin family of cell-cell adhesion proteins is abundantly expressed at mature synapses, is critical for synaptic plasticity, and is cleaved by γ-secretase in neurons upon NMDA receptor stimulation (Marambaud et al., 2003). It is, therefore, reasonable to think that a loss of synaptic cadherin cleavage by γ-secretase may lead over time to defects in synaptic plasticity and neuronal integrity and thus may contribute to the phenotype observed in these mice.

Another important question relates to the integrity in these mice of the CREB/CBP transcriptional pathway, which appeared to be significantly compromised in the PS cKO mice (Saura et al., 2004).

References:
Marambaud P, Wen PH, Dutt A, Shioi J, Takashima A, Siman R, Robakis NK. A CBP binding transcriptional repressor produced by the PS1/epsilon-cleavage of N-cadherin is inhibited by PS1 FAD mutations. Cell. 2003 Sep 5;114(5):635-45. Abstract

Saura CA, Choi SY, Beglopoulos V, Malkani S, Zhang D, Shankaranarayana Rao BS, Chattarji S, Kelleher RJ, Kandel ER, Duff K, Kirkwood A, Shen J. Loss of presenilin function causes impairments of memory and synaptic plasticity followed by age-dependent neurodegeneration. Neuron. 2004 Apr 8;42(1):23-36. Abstract

View all comments by Philippe Marambaud

In contrast with their previous paper in Neuron, Shen and colleagues now conclude that the neurodegeneration they see in both PS1 and 2 double KO mice, and nicastrin single KO mice is due to a γ-secretase defect, i.e., the loss of proteolytic function. This is part of an ongoing debate as to what extent postulated functions of presenilin outside the γ-secretase complex contribute to the overall phenotype of presenilin deficient mice. While I tend to believe that the neurodegeneration observed in their studies is indeed reflecting a real γ-secretase defect, the current paper is not conclusive in that regard. Indeed, knockout of nicastrin also destabilizes presenilin, and could theoretically affect functions of presenilin independent of its proteolytic function. However, I agree with Shen and colleagues that their current interpretation is the most likely one,...  Read more

In contrast with their previous paper in Neuron, Shen and colleagues now conclude that the neurodegeneration they see in both PS1 and 2 double KO mice, and nicastrin single KO mice is due to a γ-secretase defect, i.e., the loss of proteolytic function. This is part of an ongoing debate as to what extent postulated functions of presenilin outside the γ-secretase complex contribute to the overall phenotype of presenilin deficient mice. While I tend to believe that the neurodegeneration observed in their studies is indeed reflecting a real γ-secretase defect, the current paper is not conclusive in that regard. Indeed, knockout of nicastrin also destabilizes presenilin, and could theoretically affect functions of presenilin independent of its proteolytic function. However, I agree with Shen and colleagues that their current interpretation is the most likely one, as the major defect in nicastrin deficient cells is the loss of proteolytic activity, while some presenilin level is still maintained, which could fulfill these postulated other functions. For instance the Ca2+ leakage function is maintained in APh1 deficient cells (Tu et al., 2006) indicating that presenilin can exert that function outside of the complex.

The big question, as the authors discuss in their manuscript, is the identification of the substrate that is responsible for the neurodegenerative phenotype. This is a very difficult question to answer, given the many different substrates, and the possibility that any of the γ-secretase substrates which accumulate in the Nct knockout mice could theoretically contribute to this phenotype.

It is clear that γ-secretase as a drug target is not an easy one. We published recently that it is possible to knock out specifically the APh1B-γ-secretase in the brain of mice without neurodegenerative changes, and with the potential to clear Aβ peptide from the brain (Serneels et al., 2009). This indicates that a partial inhibition of the complex in brain is feasible and with acceptable side effects.

References:
Tu H, Nelson O, Bezprozvanny A, Wang Z, Lee SF, Hao YH, Serneels L, De Strooper B, Yu G, Bezprozvanny I. presenilins form ER Ca2+ leak channels, a function disrupted by familial Alzheimer's disease-linked mutations. Cell. 2006 Sep 8;126(5):981-93. Abstract

Serneels L, Van Biervliet J, Craessaerts K, Dejaegere T, Horré K, Van Houtvin T, Esselmann H, Paul S, Schäfer MK, Berezovska O, Hyman BT, Sprangers B, Sciot R, Moons L, Jucker M, Yang Z, May PC, Karran E, Wiltfang J, D'Hooge R, De Strooper B. gamma-Secretase heterogeneity in the Aph1 subunit: relevance for Alzheimer's disease. Science. 2009 May 1;324(5927):639-42. Abstract

View all comments by Bart De Strooper

1. Given the apparent central role of the presenilins in memory, and given the fact that loss of presenilins causes neurodegeneration, is it possible that presenilin dysfunction at least partially contributes to the neurodegenerative process in some familial forms of Alzheimer's? In my opinion, we indeed still have to learn a lot about the fundamental processes of neurodegeneration in Alzheimer's disease, and this paper contributes significantly to that aim.

2. What are the implications of the findings for drug development programs trying to target presenilin/γ-secretase? The second issue is not the main message of this paper, but obviously it is a question that will be raised by many researchers and managers in companies. My opinion is that a genetic knockout and a pharmacological modulation of a protein are two very different situations. For example, the HMGCoA reductase knockout gives a very early lethal phenotype—still statins are...  Read more

1. Given the apparent central role of the presenilins in memory, and given the fact that loss of presenilins causes neurodegeneration, is it possible that presenilin dysfunction at least partially contributes to the neurodegenerative process in some familial forms of Alzheimer's? In my opinion, we indeed still have to learn a lot about the fundamental processes of neurodegeneration in Alzheimer's disease, and this paper contributes significantly to that aim.

2. What are the implications of the findings for drug development programs trying to target presenilin/γ-secretase? The second issue is not the main message of this paper, but obviously it is a question that will be raised by many researchers and managers in companies. My opinion is that a genetic knockout and a pharmacological modulation of a protein are two very different situations. For example, the HMGCoA reductase knockout gives a very early lethal phenotype—still statins are one of the most widely used drugs in the world. Thus, I would say that it is absolutely necessary to continue with the γ-secretase inhibitor programs, especially with programs that try to develop "modulators" of the protease (like the NSIADs.) (Editor's note: see ARF related news story). It is important to find compounds that discriminate between APP processing and Notch processing. Once those are found, only clinical trials will allow us to make real decisions about the viability of these drugs. We have few options in Alzheimer's disease treatment; thus, it would be unwise to drop too quickly any potential approach for treatment.

View all comments by Bart De Strooper

After we identified PS1 as essential for γ-secretase activity (De Strooper et al., 1998) we all hoped it would be a—if not the—major therapeutic target in AD.

But in 2002 we had to report that the neuron-specific knockout of PS1 did not rescue the cognitive defects of APP mice, despite the nearly complete elimination of plaque and vascular amyloid pathology in old APPxPS1(n-/-) mice (Dewachter et al., 2002). The outcome was a complete and major surprise for us, difficult to explain and impossible to get past the referees of more than one major journal…and a major blow to the therapeutic potential of γ-secretase inhibitors in AD.

We believe that, despite the criticism on the non-physiological "total KO problem," the outcome of the paper of Saura et al., and of our 2002 paper, is as relevant now as it was then—and for more than one reason.

Inhibition of PS1—or "modulation" if so...  Read more

After we identified PS1 as essential for γ-secretase activity (De Strooper et al., 1998) we all hoped it would be a—if not the—major therapeutic target in AD.

But in 2002 we had to report that the neuron-specific knockout of PS1 did not rescue the cognitive defects of APP mice, despite the nearly complete elimination of plaque and vascular amyloid pathology in old APPxPS1(n-/-) mice (Dewachter et al., 2002). The outcome was a complete and major surprise for us, difficult to explain and impossible to get past the referees of more than one major journal…and a major blow to the therapeutic potential of γ-secretase inhibitors in AD.

We believe that, despite the criticism on the non-physiological "total KO problem," the outcome of the paper of Saura et al., and of our 2002 paper, is as relevant now as it was then—and for more than one reason.

Inhibition of PS1—or "modulation" if so preferred—will result in accumulation of CTF of APP and of a bunch of other transmembrane proteins. I asked in one of my previous comments on this site: Who is keeping tally on the substrates of γ-secretase? At least for the β-CTF (C99) of APP, we know they are potentially as neurotoxic as the amyloid peptides, and probably even more, since they remain attached to, and concentrated in the neurons in which they are produced. We do not know much about the (non-) physiological repercussions of remnants of other substrates of γ-secretase, but their accumulation can be safely predicted to be "not good for your brain."

The prevention of formation of AICD by inhibition of γ-secretase can or should be added to the drawbacks, now even more than in 2002, given the most recent evidence that AICD actually regulates expression of neprilysin (Pardossi-Piquard et al., 2005), that, as we all know, is a major Aβ killer!

Does that imply that the γ-secretase complex is off-bounds as a therapeutic target for AD as we advocated before (Dewachter and Van Leuven, 2002) based on our 2002 data? I believe so, but not being clairvoyant, I cannot but leave the question open. Nevertheless, the structural and functional complexity of γ-secretase, the inherent and not understood control of its activity and specificity, combined with the disparity of its substrates in neurons and in many (most?) other cell types in our body, is so overwhelming that finding weak spots or leap holes in its armor is a daunting task.

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 De Strooper B, Saftig P, Craessaerts K, Vanderstichele H, Guhde G, Annaert W, Von Figura K, Van Leuven F. Deficiency of presenilin-1 inhibits the normal cleavage of amyloid precursor protein. Nature. 1998 Jan 22;391(6665):387-90. Abstract Dewachter I, Van Leuven F. Secretases as targets for the treatment of Alzheimer's disease: the prospects. Lancet Neurol. 2002 Nov;1(7):409-16. Review. Abstract Pardossi-Piquard R, Petit A, Kawarai T, Sunyach C, Alves da Costa C, Vincent B, Ring S, D'Adamio L, Shen J, Muller U, St George Hyslop P, Checler F. Presenilin-dependent transcriptional control of the Aβ-degrading enzyme neprilysin by intracellular domains of βAPP and APLP. Neuron. 2005 May 19;46(4):541-54. Abstract

View all comments by Fred Van Leuven

I am impressed that these animals develop neurological problems, reminiscent of clinical AD, without the accumulation of vast amounts of extracellular deposits of Aβ peptides. One has to wonder whether the earliest forms of the human disease share these characteristics, with the intracellular accumulation of as yet unidentified toxic APP products preceding the development of amyloidosis.

View all comments by Vincent Marchesi

Hyoung-gon Lee, Xiongwei Zhu, George Perry, Mark Smith

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.

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. Abstract.

Nunomura A, Perry G, Aliev G, Hirai K, Takeda A, Balraj EK, Jones PK, Ghanbari H, Wataya T, Shimohama S, Chiba S, Atwood CS, Petersen RB, Smith MA. Oxidative damage is the earliest event in Alzheimer disease. J Neuropathol Exp Neurol. 2001 Aug;60(8):759-67. Abstract.

Nunomura A, Chiba S, Lippa CF, Cras P, Kalaria RN, Takeda A, Honda K, Smith MA, Perry G. Neuronal RNA oxidation is a prominent feature of familial Alzheimer's disease. Neurobiol Dis. 2004 Oct;17(1):108-13. Abstract.

Rottkamp CA, Atwood CS, Joseph JA, Nunomura A, Perry G, Smith MA. The state versus amyloid-beta: the trial of the most wanted criminal in Alzheimer disease. Peptides. 2002 Jul;23(7):1333-41. 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
View all comments by George Perry
View all comments by Mark A. Smith
View all comments by Xiongwei Zhu

Discrepancies in Two Recent Papers on ER Ca2+-leak Channels in Presenilin1, -2 Double Knockout Cells
This paper describes presenilin (PS)-related mechanisms that affect Ca2+ leak from the endoplasmic reticulum (ER). However, it points to a very different mechanism—Ca2+-channel leak properties of presenilin—to that which we have recently published: upregulation of type 1 inositol 1,4,5-trisphosphate receptor (IP3R1) (Kasri et al., 2006). Although these two conclusions are not mutually exclusive, the niggling point is that both papers report very different and even sometimes opposing experimental findings. There is no obvious explanation for these discrepancies, but it is clear that all methodologies currently applied to evaluate ER Ca2+ concentrations and ER Ca2+ leak are imperfect and often lead to contradictory results. This was extensively discussed by Clark Distelhorst and Gordon Shore in their recent review of the conflicting findings...  Read more

Discrepancies in Two Recent Papers on ER Ca2+-leak Channels in Presenilin1, -2 Double Knockout Cells
This paper describes presenilin (PS)-related mechanisms that affect Ca2+ leak from the endoplasmic reticulum (ER). However, it points to a very different mechanism—Ca2+-channel leak properties of presenilin—to that which we have recently published: upregulation of type 1 inositol 1,4,5-trisphosphate receptor (IP3R1) (Kasri et al., 2006). Although these two conclusions are not mutually exclusive, the niggling point is that both papers report very different and even sometimes opposing experimental findings. There is no obvious explanation for these discrepancies, but it is clear that all methodologies currently applied to evaluate ER Ca2+ concentrations and ER Ca2+ leak are imperfect and often lead to contradictory results. This was extensively discussed by Clark Distelhorst and Gordon Shore in their recent review of the conflicting findings regarding the effects of Bcl-2 proteins on ER Ca2+ (Distelhorst and Shore 2004).

Before analyzing potential reasons why different experimental findings have been obtained in these two presenilin papers, I will first summarize the findings in each that are not disproved by findings in the other.

The most important observation in our paper is that there is an isoform-specific fourfold upregulation of IP3R1 in murine embryonic fibroblast (MEF) PS double knockout (dko) cells. This observation was very solid, as it was done not only using isoform-specific antibodies against both isoforms of IP3R, but also using a common antibody that allows a simultaneous detection of both receptor isoforms. The latter allows a determination of the relative expression of IP3R-isoforms and is therefore independent of load controls that are problematic when comparing different cell types. One such control, actin, can be especially problematic because the dko cells are characterized by quite a different cellular morphology. Moreover, the fact that the enhanced Ca2+ leak could be reversed using siRNA-mediated downregulation of specifically IP3R1 demonstrates that the increased level in IP3R1 is the primary cause of this leak. The role of IP3R1 as an ER Ca2+-leak channel is in agreement with findings from other groups (Oakes et al., 2005).

The major observation in the paper by Tu et al. is that wild-type presenilins, but not PS1-M146V and PS2-N141I FAD mutants, can form low-conductance divalent-cation-permeable ion channels in planar lipid bilayers. These channel properties of presenilins were confirmed using lipid bilayer reconstitution of the purified proteins, and it suggests a Ca2+ signaling function for presenilins which would provide further support for the “Ca2+ hypothesis of AD.”

While the basic observations of both papers may point to two of the potential mechanisms for ER Ca2+ leak, it should be clear that many other leak pathways may coexist, and, as was adequately discussed by Tu et al., the exact identity of ER Ca2+ leak channels still largely remains an “enigma of Ca2+ signaling” (Camello et al., 2002).

This is where the deviating observations and conflicting results come in. Even worse, the basic observation about the ER Ca2+ level is exactly the opposite in both papers. Although essentially the same immortalized mouse embryonic cell lines (MEF and MEF dko fibroblasts) were used, we found that MEF PS dko cells had a lower ER Ca2+ level, explained by increased IP3R1-mediated leak, whereas in the Tu et al. paper the opposite was found—as may be expected if the leak occurs via presenilins, which have been knocked out. There is no explanation for this discrepancy except that different techniques were used to measure ER (Ca2+) and the Ca2+ leak. In our hands, targeted aequorins were used to estimate ER (Ca2+) and saponin-permeabilized monolayers were used for estimating the leak rate. In the Tu et al. paper, Mag-Fura was used for ER Ca2+ measurement, and Fura-2 fluorescence was used for evaluating Ca2+ fluxes in microsomes.

It is very clear that all these methods have their own drawbacks, and the main problem would appear to be what fraction of the ER is actually measured. In each of the methods used there are uncertainties as to whether only the ER is targeted and whether it may be disturbed by the preparation procedures. As a result it may very well be that different subfractions of the ER have been evaluated. The depletion of the ER in the aequorin method or the use of saponin may have affected the structure of the ER. The preparation of microsomes, on the other hand, certainly results in a mixture of membrane fractions, the distribution and purity of which may also be different for different cell types. A second drawback in these studies is the means used to deplete the ER: ionomycin is not specific and will deplete all Ca2+-containing compartments, whereas thapsigargin will target only those compartments filled up by the SERCA-type Ca2+ pump. In our work, the saponin-permeabilized monolayers largely reflect the thapsigargin-dependent Ca2+ stores, and the small thapsigargin-independent part was subtracted in the calculations. For the preparations in the Tu et al. paper, both for intact cells and for microsomes, large differences were observed between ionomycin-releasable and thapsigargin-releasable Ca2+. These data are interpreted as a measure of the total Ca2+ content and the rate of Ca2+ release, respectively. It is, however, not established that the ionomycin-derived Ca2+ content only reflects the ER. Furthermore, the thapsigargin-induced Ca2+ release rate in intact cells may not only reflect ER Ca2+ release but also the rate of Ca2+ efflux driven by PMCA or Na+/Ca2+ exchanger and Ca2+ uptake by the thapsigargin-independent stores (Golgi, mitochondria). Moreover, in microsomal preparations, the Ca2+-release rate will not only depend on the distribution of presenilin in the different microsomal fractions but also on their diameter, composition, and aggregation, and these parameters may be variable if preparations have to be made from different cell types. One should keep in mind that the MEF dko cells are defective cells that grow more slowly and have different morphology as compared to the wild-type cells. This may result in microsomal fractions with different biochemical and physical properties.

In conclusion, both papers have provided evidence for new mechanisms of ER Ca2+ control, and these mechanisms are clearly related to presenilin expression and may therefore play a role in the pathology of AD. However, the quantitative significance of these leak pathways in intracellular compartments, and particularly in different ER fractions, is very difficult to evaluate. This is largely because there are no fool-proof methods for obtaining preparations that truly reflect and measure the properties of the ER in a real cellular context. Moreover, the cellular heterogeneity of the ER and the existence of other membrane compartments, where IP3Rs or presenilins may operate, remain difficult to fully appreciate. Finally, the molecular tools to evoke ER-related Ca2+ fluxes are imperfect and not equally reliable in all conditions. Appreciation of the significance of the above-mentioned mechanisms for neuronal function and dysfunction will have to wait until more adequate techniques for measuring cellular Ca2+ signals are available.

View all comments by Humbert De Smedt

The planar lipid bilayer approach was, therefore, an ideal preparation to start addressing presenilin function in membranes and its relation to the Ca2+ signaling dysregulation seen with certain AD-linked presenilin mutations. This technique allows one to insert specific channels of interest into a...  Read more

The planar lipid bilayer approach was, therefore, an ideal preparation to start addressing presenilin function in membranes and its relation to the Ca2+ signaling dysregulation seen with certain AD-linked presenilin mutations. This technique allows one to insert specific channels of interest into a modified “model membrane” in order to observe and manipulate their function. Given that wild-type presenilin can form cation-permeable channels in these lipid bilayer models—and that the PS1-M146V and PS2-N141I mutants are impaired in this function—the extension to biological models using murine embryonic fibroblasts (MEFs) and rescue experiments in PS double knockouts (PS-DKOs) becomes easier to interpret and certainly more powerful. Hypothesizing that ER stores overfill due to impaired Ca2+ leak current through presenilin channels is a novel proposition, and it is backed up by clear mechanistic evidence in both model membranes and biological systems.

This study is particularly elegant in that it contributes to our understanding of presenilin function at several levels. At the basic science level, we have new insight into the role of presenilin in the ER—why it is even located there (addressing the spatial paradox)—and a novel candidate for the leak channel—which has been inferred but never really seen. And, since the leak function is separate from its role in the γ-secretase complex, these results also imply an additional, separate, and parallel role of the presenilins in maintaining Ca2+ homeostasis. At the neuropathology level, this is the first real mechanistic study that can point to how AD-linked presenilin mutations can result in increased ER Ca2+ stores through a loss of function.

At a more global level, there is still much to be explored regarding how mutant PS and ER Ca2+ signaling dysregulations are linked to the pathophysiology of AD. Primarily, can impaired Ca2+ leak channels be linked to Aβ plaque formation and neurofibrillary tangles that are diagnostic of AD, or are they a separate and independent phenomenon in the disease process? Interestingly, in Tu’s study, not all PS mutations generated the same channel phenotype, and this will ultimately need reconciling. The PS1-δE9 mutation resulted in an apparent gain of function with increased cation conductance—yet in humans, the M146V and δE9 mutations ultimately result in the same disease state. In the basic research realm, an additional point that needs reconciling is the conflicting data regarding SERCA pump blockers (e.g., thapsigargin). In several studies examining effects of mutant PS1, application of SERCA blockers results in enhanced Ca2+ release into the cytosol (Guo et al., 1997; Leissring et al., 2000; Herms et al., 2003; Stutzmann, personal observation in brain slice preparations), which is at odds with the proposed reduction in the PS-leak channel conductance and the raw data presented in the Tu et al., study.

Determining if/how presenilin interacts with other ER Ca2+ channels such as the IP3 and ryanodine receptors is an important next step, particularly in light of several recent studies demonstrating an increase in ryanodine receptor number and function in PS1-M146V expressing neurons (Chan et al., 2000; Smith et al., 2005; Stutzmann et al., 2006). So, there is likely still more to the PS story that has yet to be uncovered, but this study provides vital information to both the basic science and AD fields, and infuses new life into the Ca2+ hypothesis of AD. And, perhaps most importantly, it provides a clear new direction with which to focus future PS-Ca2+ signaling studies.

References:
Guo Q, Sopher BL, Furukawa K, Pham DG, Robinson N, Martin GM, Mattson MP. Alzheimer's presenilin mutation sensitizes neural cells to apoptosis induced by trophic factor withdrawal and amyloid beta-peptide: involvement of calcium and oxyradicals. J Neurosci. 1997 Jun 1 ; 17(11):4212-22. Abstract

Leissring MA, Akbari Y, Fanger CM, Cahalan MD, Mattson MP, LaFerla FM. Capacitative calcium entry deficits and elevated luminal calcium content in mutant presenilin-1 knockin mice. J Cell Biol. 2000 May 15 ; 149(4):793-8. Abstract

Herms J, Schneider I, Dewachter I, Caluwaerts N, Kretzschmar H, Van Leuven F. Capacitive calcium entry is directly attenuated by mutant presenilin-1, independent of the expression of the amyloid precursor protein. J Biol Chem. 2003 Jan 24 ; 278(4):2484-9. Abstract

Chan SL, Mayne M, Holden CP, Geiger JD, Mattson MP. Presenilin-1 mutations increase levels of ryanodine receptors and calcium release in PC12 cells and cortical neurons. J Biol Chem. 2000 Jun 16 ; 275(24):18195-200. Abstract

Smith IF, Green KN, LaFerla FM. Calcium dysregulation in Alzheimer's disease: recent advances gained from genetically modified animals. Cell Calcium. 2005 Sep-Oct ; 38(3-4):427-37. Abstract

Stutzmann GE, Smith I, Caccamo A, Oddo S, LaFerla FM, Parker I. Enhanced ryanodine receptor recruitment contributes to Ca2+ disruptions in young, adult, and aged Alzheimer's disease mice. J Neurosci. 2006 May 10 ; 26(19):5180-9. Abstract

View all comments by Grace (Beth) Stutzmann

The first finding obtained by Bezprozvanny and colleagues, showing that PSs are leak channels, does not contradict our published data: we have repeatedly demonstrated that overexpression of wt-PS2 and, to a lesser extent, also of wt-PS1, reduces the ER Ca2+ level in different cell models (Zatti et al., 2004; Giacomello et al., 2005;...  Read more

The first finding obtained by Bezprozvanny and colleagues, showing that PSs are leak channels, does not contradict our published data: we have repeatedly demonstrated that overexpression of wt-PS2 and, to a lesser extent, also of wt-PS1, reduces the ER Ca2+ level in different cell models (Zatti et al., 2004; Giacomello et al., 2005; Zatti et al., 2006). Surprisingly, what is not consistent with our findings is the fact that, in our models, the expression of various FAD-linked PS mutants often results in a “gain of function” if considering the effect of PSs on the ER leakage. In fact, the ability of wt PSs to reduce ER Ca2+ release is also shared by different PS mutants: PS2-M239I (Zatti et al., 2004); PS2-T122R (Giacomello et al., 2005); PS2-N141I, PS2-D366A, PS1-A246E, PS1-M146L, PS1-P117L (Zatti et al., 2006). Notably, these mutations include two mentioned by Bezprozvanny and colleagues (PS2-N141I and PS1-M146L/V), as well as one devoid of γ-secretase activity (PS2-D366A).

We have published data (Zatti et al., 2006) showing that the store Ca2+ content is unchanged or even reduced when PSs are expressed in different cell models either stably (such as in human FAD fibroblasts, HEK293, and SH-SY5Y clones) or transiently (such as in HeLa and SH-SY5Y cells, MEFs, and primary cultures of rat neurons). These results were obtained by using two different methodological approaches, that is, by cytosolic Ca2+ imaging with fura-2 (as described by Bezprozvanny and colleagues) and by recombinant ER-targeted aequorin (as described by Kasri et al., 2006). No evidence of an exaggerated Ca2+ release was found in cells expressing any of the investigated PS2 (M239I, -T122R, -N141I, -D366A) or PS1 (-A246E, -L286V, -M146L, -P117L) mutations. Similarly, no Ca2+ overload was found when directly measuring ER and Golgi apparatus Ca2+ levels (using appropriately targeted aequorins) in HeLa and SH-SY5Y cells overexpressing the above-mentioned PS mutants (Zatti et al., 2006), as well as in the stable clones HEK293/PS1-M146L and SH-SY5Y/PS2-T122R and in DKO MEF cells (our unpublished data, and see also Kasri et al., 2006). Consistently, DKO MEFs did not show an increased Ca2+ store content if compared to MEFs expressing only the wt-PS1 when measuring the cytosolic Ca2+ changes induced by store depletion with cyclopiazonic acid (Zatti et al., 2006).

Thus, the reasons for these discrepancies cannot merely be due to differences in the methodology employed for Ca2+ measurements. The true reasons for such discrepancies should indeed be sought if one wishes to shed light on this complex phenomenon. Conversely, ignoring them does not help the AD community and, more importantly, hinders scientific progress.

We believe that, among the different models employed in this type of investigation, human fibroblasts from FAD patients should be given at least the same weight as MEFs, not least because we are interested in the human pathology. A reduced and not an exaggerated Ca2+ release was detected by cytosolic fura-2 measurements in human FAD-fibroblasts carrying the PS1-M146L (two patients) or the PS1-P117L (one patient), whose donors were presenting a devastating early-age-of-onset AD (30 years for the PS1-P117L-carrying subject; Zatti et al., 2006). A stronger reduction in ER Ca2+ content was inferred with the same technique in human FAD-fibroblasts carrying the PS2-M239I (two patients) or the PS2-T122R (two patients) when compared to healthy age-matched control subjects (Zatti et al., 2004; Giacomello et al., 2005).

It is also worth noting that the “abnormal Ca2+ signaling” usually reported for human FAD fibroblasts not always means an increased Ca2+ load since a reduced Ca2+ release was also observed (Peterson et al., 1988; McCoy et al., 1993). The discussion on this issue is further complicated by the fact that the large majority of the studies with AD fibroblasts were carried out in the 1980s-1990s when Alzheimer donors were not genetically characterized. Interestingly, by using aequorin, McCoy et al. (1993) showed a reduced Ca2+release in human early-onset FAD fibroblasts from a Canadian family which was recently shown to carry the PS1-A246E mutation (Huang et al., 2005).

Furthermore, we have to consider that, in FAD fibroblasts, at variance with the rescued DKO MEFs, the PS mutant exerts its effect in the presence of the endogenous wild-type proteins, as occurring also in the majority of the cell models tested. This fact makes the comparison even more problematic.

Given the suggested protective role exerted by a low ER Ca2+ level (Scorrano et al., 2003), we proposed that PS mutations which strongly reduce the ER Ca2+ content (such as those in PS2) should attenuate the pathology, whereas other mutations that leave the ER Ca2+ content unchanged or mildly reduced (such as those in PS1) could be unable to compensate for other defects due to the mutations themselves. Indeed, oxidative stress induces a pronounced Ca2+ overload in PS1-A246E-FAD fibroblasts compared to aged controls (Huang et al., 2005). Our hypothesis is thus consistent with the later ages of onset and milder AD phenotypes observed in patients carrying PS2 mutations with respect to those carrying PS1 ones.

In summary, as far as the physiological role of PSs is concerned, our results are in agreement with those reached by Bezprozvanny and colleagues. However, the presence of contrasting findings with the pathological mutations shows the limitations of the simple definition “gain or loss of function” for multifaceted proteins such as PSs, especially when considering how different are the backgrounds in which their effects are evaluated.

References:
Giacomello M, Barbiero L, Zatti G, Squitti R, Binetti G, Pozzan T, Fasolato C, Ghidoni R, Pizzo P. Reduction of Ca2+ stores and capacitative Ca2+ entry is associated with the familial Alzheimer's disease presenilin-2 T122R mutation and anticipates the onset of dementia. Neurobiol Dis. 2005 Apr;18(3):638-48. Abstract

Kasri NN, Kocks SL, Verbert L, Hebert SS, Callewaert G, Parys JB, Missiaen L, De Smedt H. Up-regulation of inositol 1,4,5-trisphosphate receptor type 1 is responsible for a decreased endoplasmic-reticulum Ca2+ content in presenilin double knock-out cells. Cell Calcium. 2006 Jul;40(1):41-51. Epub 2006 May 3. Abstract

Huang HM, Chen HL, Xu H, Gibson GE. Modification of endoplasmic reticulum Ca2+ stores by select oxidants produces changes reminiscent of those in cells from patients with Alzheimer disease. Free Radic Biol Med. 2005 Oct 15;39(8):979-89. Abstract

McCoy KR, Mullins RD, Newcomb TG, Ng GM, Pavlinkova G, Polinsky RJ, Nee LE, Sisken JE. Serum- and bradykinin-induced calcium transients in familial Alzheimer's fibroblasts. Neurobiol Aging. 1993 Sep-Oct;14(5):447-55. Abstract

Peterson C, Ratan RR, Shelanski ML, Goldman JE. Altered response of fibroblasts from aged and Alzheimer donors to drugs that elevate cytosolic free calcium. Neurobiol Aging. 1988 May-Jun;9(3):261-6. Abstract

Scorrano L, Oakes SA, Opferman JT, Cheng EH, Sorcinelli MD, Pozzan T, Korsmeyer SJ. BAX and BAK regulation of endoplasmic reticulum Ca2+: a control point for apoptosis. Science. 2003 Apr 4;300(5616):135-9. Epub 2003 Mar 6. Abstract

Smith IF, Green KN, LaFerla FM. Calcium dysregulation in Alzheimer's disease: recent advances gained from genetically modified animals. Cell Calcium. 2005 Sep-Oct;38(3-4):427-37. Review. Abstract

Zatti G, Ghidoni R, Barbiero L, Binetti G, Pozzan T, Fasolato C, Pizzo P. The presenilin 2 M239I mutation associated with familial Alzheimer's disease reduces Ca2+ release from intracellular stores. Neurobiol Dis. 2004 Mar;15(2):269-78. Abstract

Zatti G, Burgo A, Giacomello M, Barbiero L, Ghidoni R, Sinigaglia G, Florean C, Bagnoli S, Binetti G, Sorbi S, Pizzo P, Fasolato C. Presenilin mutations linked to familial Alzheimer's disease reduce endoplasmic reticulum and Golgi apparatus calcium levels. Cell Calcium. 2006 Jun;39(6):539-50. Epub 2006 Apr 18. Abstract

View all comments by Giuliano Binetti
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View all comments by Sandro Sorbi

I performed aluminum neurotoxicity experiments on hippocampal rat neurons several years ago and found dantrolene and dimethylsulfoxide reduced cell death from aluminum toxicity, indicating aluminum toxicity may be mediated through release of calcium from intracellular stores and oxidative stress (1).

There may be multiple mechanisms disrupting calcium metabolism in the endoplasmic reticulum, including metals such as aluminum and other metals potentially capable of oxidation such as copper and iron. Oxidative...  Read more

I performed aluminum neurotoxicity experiments on hippocampal rat neurons several years ago and found dantrolene and dimethylsulfoxide reduced cell death from aluminum toxicity, indicating aluminum toxicity may be mediated through release of calcium from intracellular stores and oxidative stress (1).

There may be multiple mechanisms disrupting calcium metabolism in the endoplasmic reticulum, including metals such as aluminum and other metals potentially capable of oxidation such as copper and iron. Oxidative stress might also be implicated as well.

I am not sure how β amyloid could effect calcium metabolism within the endoplasmic reticulum and other intracellular stores, but amyloid precursor protein could be implicated as well, since it may be capable of forming ion channels.

Disruption of calcium metabolism and β amyloid toxicity may act synergistically in causing cellular dysfunction and Alzheimer disease.

If intracellular structures such as endoplasmic reticulum and sarcoplasmic reticulum are effected by disturbed calcium metabolism, protein assembly in intracellular structures may result in dysfunctional proteins unable to perform intracellular processes normally with subsequent cellular death and resulting Alzheimer disease.

References:
Brenner S. Aluminum neurotoxicity is reduced by dantrolene and dimethylsulfoxide in cultured rat hippocampal neurons. Biol Trace Elem Res. 2002 Apr; 86 (1) 85-89. Abstract

View all comments by Steven Brenner

Under resting conditions, steady-state [Ca2+]L is determined by the dynamic equilibrium of two components: an active Ca2+ uptake mediated by ATP-dependent Ca2+ pumps of the SERCA family and passive Ca2+ efflux via leak channels. Even though this pump-leak cycle appears to be a common property of Ca2+-storing organelles, little is known about the proteins controlling the Ca2+ leak pathway. Several...  Read more

Under resting conditions, steady-state [Ca2+]L is determined by the dynamic equilibrium of two components: an active Ca2+ uptake mediated by ATP-dependent Ca2+ pumps of the SERCA family and passive Ca2+ efflux via leak channels. Even though this pump-leak cycle appears to be a common property of Ca2+-storing organelles, little is known about the proteins controlling the Ca2+ leak pathway. Several mechanisms involving quite different proteins have been previously suggested to explain the basal Ca2+ leak from ER, namely: 1) reverse Ca2+ flux through the pumps (Toyoshima et al., 2002); 2) Ca2+ leak in neutral complexes with small molecules by translocon channels (Lomax et al., 2002; Van Coppenolle et al., 2004); 3) the fluxes of Ca2+ through “natural” ionophores, such as bile acids (Zimniak et al., 1991); 4) an anti-apoptotic protein Bcl-2–mediated Ca2+ leak (Bassik et al., 2004); 5) IP3R- or RYR-mediated Ca2+ leak (Oakes et al., 2005) and, more recently, 6) pannexin 1-mediated calcium leak (Vanden Abeelle et al., 2006). However, “the drawing of these mechanisms is only a fantasy map of the leak terra incognita, and discovery of the exact mechanisms of calcium leak remains a challenge to scientists working in the calcium signaling field.” (Camello et al., 2002).

The team of Ylya Bezprozvanny, using a multidisciplinary approach, clearly demonstrates that the nine transmembrane domain ER proteins, presenilins, account for almost 80 percent of passive calcium leak from the ER. The results of their study strongly suggest that presenilins can form calcium-permeable ion channels and, moreover, that the genetic deletion of presenilins (in double knockout, DKO, mice) resulted in a sixfold reduction in the rate of calcium leak across ER membrane. Heterologous expression of presenilins in DKO mouse embryonic fibroblasts was able to rescue calcium leakage defects observed in DKO cells, which is consistent with an ion “leak-channel” function of presenilins in the ER membrane.

Even more intriguing is the finding that presenilin mutants associated with familial Alzheimer disease (FAD) were not able to form calcium leak channels. Thus, the results of this study strongly support the hypothesis of a crucial role of calcium homeostasis in Alzheimer disease, pointing out the specific function of presenilins.

References:
Bassik MC, Scorrano L, Oakes SA, Pozzan T, Korsmeyer SJ. Phosphorylation of BCL-2 regulates ER Ca2+ homeostasis and apoptosis. EMBO J. 2004 Mar 10;23(5):1207-16. Epub 2004 Mar 4. Abstract

Camello C, Lomax R, Petersen OH, Tepikin AV. Calcium leak from intracellular stores--the enigma of calcium signalling. Cell Calcium. 2002 Nov-Dec;32(5-6):355-61. Review. Abstract

Lomax RB, Camello C, Van Coppenolle F, Petersen OH, Tepikin AV. Basal and physiological Ca(2+) leak from the endoplasmic reticulum of pancreatic acinar cells. Second messenger-activated channels and translocons. J Biol Chem. 2002 Jul 19;277(29):26479-85. Epub 2002 May 6. Abstract

Toyoshima C, Nomura H. Structural changes in the calcium pump accompanying the dissociation of calcium. Nature. 2002 Aug 8;418(6898):605-11. Abstract

Vanden Abeele F, Skryma R, Shuba Y, Van Coppenolle F, Slomianny C, Roudbaraki M, Mauroy B, Wuytack F, Prevarskaya N. Bcl-2-dependent modulation of Ca(2+) homeostasis and store-operated channels in prostate cancer cells. Cancer Cell. 2002 Mar;1(2):169-79. Abstract

Vanden Abeele F, Bidaux G, Gordienko D, Beck B, Panchin YV, Baranova AV, Ivanov DV, Skryma R, Prevarskaya N. Functional implications of calcium permeability of the channel formed by pannexin 1. J Cell Biol. 2006 Aug 14;174(4):535-46. Abstract

Van Coppenolle F, Vanden Abeele F, Slomianny C, Flourakis M, Hesketh J, Dewailly E, Prevarskaya N. Ribosome-translocon complex mediates calcium leakage from endoplasmic reticulum stores. J Cell Sci. 2004 Aug 15;117(Pt 18):4135-42. Epub 2004 Jul 27. Abstract

Zimniak P, Little JM, Radominska A, Oelberg DG, Anwer MS, Lester R. Taurine-conjugated bile acids act as Ca2+ ionophores. Biochemistry. 1991 Sep 3;30(35):8598-604. Abstract

View all comments by Natalia Prevarskaya

Binetti and colleagues raise interesting questions about the effects of presenilin FAD mutations on ER Ca2+ content and on inositol trisphosphate receptor (InsP3R)-mediated Ca2+ release. We attempted to reconcile our results with that of Zatti at al.; however, we ran into significant difficulties in interpreting their data.

Let us consider an example of two PS1 FAD mutants for which extensive datasets are available from several laboratories. Zatti et al. reported that expression of PS1-M146L resulted in reduced Ca2+ response to cyclopiazonic acid (CPA) + histamine (Fig. 1C), no change in response to CPA + bradykinin (BK) (Fig. 1B), ...  Read more

Binetti and colleagues raise interesting questions about the effects of presenilin FAD mutations on ER Ca2+ content and on inositol trisphosphate receptor (InsP3R)-mediated Ca2+ release. We attempted to reconcile our results with that of Zatti at al.; however, we ran into significant difficulties in interpreting their data.

Let us consider an example of two PS1 FAD mutants for which extensive datasets are available from several laboratories. Zatti et al. reported that expression of PS1-M146L resulted in reduced Ca2+ response to cyclopiazonic acid (CPA) + histamine (Fig. 1C), no change in response to CPA + bradykinin (BK) (Fig. 1B), no change for response to CPA + carbamylcholine (CaCh) in HEK293 stable lines (Fig. 3B), no change in ER Ca2+ levels (Fig. 4C) and reduced Golgi Ca2+ levels (Fig. 5C). They also report that CPA response was reduced in human PS1-M146L fibroblasts (Fig. 1D). The response in neurons transfected with PS1-M146L was not tested in Zatti at al. paper.

It is hard to reach a conclusion from these results about the effect of PS1-M146L on ER Ca2+ signaling. Most data in Zatti at al. suggest that PS1-M146L has either no effect or reduces the ER Ca2+ content and the InsP3R-mediated Ca2+ release. This conclusion directly contradicts our results with transfected DKO MEF cells [2] and the extensive characterization of the effects of PS1-M146V mutant on the InsP3R-mediated Ca2+ signals in Xenopus oocytes and in hippocampal neurons by Parker and La Ferla’s laboratories [3,4].

A similar situation exists for PS1-A246E FAD mutant. Zatti et al. reported that expression of PS1-A246E resulted in reduced Ca2+ response to CPA + histamine (Fig. 1C), reduced Ca2+ response to CPA + BK (Fig. 2B), reduced ER Ca2+ levels in HeLa cells, but unchanged ER Ca2+ levels in SH-SY5Y cells (Fig. 4C), reduced Golgi Ca2+ levels in both HeLa and SH-SY5Y (Fig. 5C), and no change in CaCh-induced response in transfected rat cortical neurons (Fig. 6D).

Once again, most of these results seem to indicate that PS1-A246E has either no effect or reduces the ER Ca2+ content and the InsP3R-mediated Ca2+ release. This conclusion directly contradicts our results with transfected DKO MEF cells and human A246E fibroblasts [5], and results from studies of the InsP3R-mediated Ca2+ signals in hippocampal neurons from A246E transgenic mouse performed by Jochen Herms’s laboratory [6].

In summary, we agree with Giuliano Binetti and colleagues that effects of FAD mutations in presenilins on ER Ca2+ homeostasis is a very exciting and important area of AD research. However, as it is clear from the above discussion, much additional work by many laboratories will be required to clarify the exact nature of this interesting phenomenon.

References:
1. Zatti G, Burgo A, Giacomello M, Barbiero L, Ghidoni R, Sinigaglia G, Florean C, Bagnoli S, Binetti G, Sorbi S, Pizzo P, Fasolato C. Presenilin mutations linked to familial Alzheimer's disease reduce endoplasmic reticulum and Golgi apparatus calcium levels. Cell Calcium. 2006 Jun;39(6):539-50. Epub 2006 Apr 18. Abstract

2. Tu H, Nelson O, Bezprozvanny A, Wang Z, Lee SF, Hao YH, Serneels L, De Strooper B, Yu G, Bezprozvanny I. Presenilins form ER Ca2+ leak channels, a function disrupted by familial Alzheimer's disease-linked mutations. Cell. 2006 Sep 8;126(5):981-93. Abstract

3. Leissring MA, Paul BA, Parker I, Cotman CW, LaFerla FM. Alzheimer's presenilin-1 mutation potentiates inositol 1,4,5-trisphosphate-mediated calcium signaling in Xenopus oocytes. J Neurochem. 1999 Mar;72(3):1061-8. Abstract

4. Stutzmann GE, Caccamo A, LaFerla FM, Parker I. Dysregulated IP3 signaling in cortical neurons of knock-in mice expressing an Alzheimer's-linked mutation in presenilin1 results in exaggerated Ca2+ signals and altered membrane excitability. J Neurosci. 2004 Jan 14;24(2):508-13. Abstract

5. Nelson O, Tu H, Lei T, Bentahir M, de Strooper B and Bezprozvanny I. (2006) Familial Alzheimer's disease mutations in presenilin 1 disrupt endoplasmic reticulum calcium leak function. Submitted.

6. Schneider I, Reverse D, Dewachter I, Ris L, Caluwaerts N, Kuiperi C, Gilis M, Geerts H, Kretzschmar H, Godaux E, Moechars D, Van Leuven F, Herms J. Mutant presenilins disturb neuronal calcium homeostasis in the brain of transgenic mice, decreasing the threshold for excitotoxicity and facilitating long-term potentiation. J Biol Chem. 2001 Apr 13;276(15):11539-44. Epub 2001 Jan 23. Abstract

View all comments by Ilya Bezprozvanny

In my opinion, the research by Bezprozvanny and colleagues emphasizes the importance of the fundamental role of free calcium modifications in cells undergoing biochemical changes underlying AD. While not questioning the key role of Aβ peptides in this disease, the data add another possible dimension to the key role performed by calcium in cellular stress and death following the biochemical modifications characterizing AD. Hence, some presenilin mutations affecting γ-secretase activity can impair cell viability by increasing Aβ peptide production or by shifting the latter towards the more amyloidogenic Aβ42, resulting in Aβ oligomerization and cell membrane(s) permeabilization. Other mutations that do...  Read more

In my opinion, the research by Bezprozvanny and colleagues emphasizes the importance of the fundamental role of free calcium modifications in cells undergoing biochemical changes underlying AD. While not questioning the key role of Aβ peptides in this disease, the data add another possible dimension to the key role performed by calcium in cellular stress and death following the biochemical modifications characterizing AD. Hence, some presenilin mutations affecting γ-secretase activity can impair cell viability by increasing Aβ peptide production or by shifting the latter towards the more amyloidogenic Aβ42, resulting in Aβ oligomerization and cell membrane(s) permeabilization. Other mutations that do not affect γ-secretase activity can disrupt calcium leakage from the ER, resulting in increased calcium stores in the ER and subsequent ER stress. It cannot be excluded that, for some specific PS mutations, the two effects may act synergistically with more severe cellular stress.

View all comments by Massimo Stefani