This study shows that Aβ1-40 (as well as PrP106-126 peptide) induces ER stress, leading to apoptotic death in neurons. Previous studies have ruled out the primary role of ER stress in AD (e.g., Piccini et al., 2004). It would be interesting to ascertain if endogenous Aβ (produced through a Bri/Aβ fusion protein, e.g.) induces the same cascade of events described in the study. Then, check if Aβ1-42 has the same effects. Moreover, I would test the effect of different states of aggregation of Aβ peptides.

View all comments by Massimo Tabaton

Our lab previously reported activation of the UPR in AD neurons (Hoozemans et al., 2005). In the current paper, Ferreiro et al. show induction of BiP levels, as well as decreased pro-caspase-12 levels induced by Aβ1-40. This may indicate that the ER stress response (including the apoptotic branch of the UPR) is activated directly by Aβ, and may be the cause of the UPR activation that we observe in AD neurons. However, the data obtained by Ferreiro et al. in vitro appear not to corroborate fully with observations from the actual patient material. The data presented in the Ferreiro paper suggest that apoptotic cell death is a direct consequence of Aβ-induced UPR activation, whereas we find no evidence of apoptosis in AD neurons with an activated UPR. The UPR is activated as a protective mechanism to restore ER homeostasis, and although it can result in cell death after prolonged activation, it is not necessarily a bad thing. This is in agreement with our observation that the UPR is activated relatively early in AD pathology. In this respect it would be interesting to distinguish...  Read more

Our lab previously reported activation of the UPR in AD neurons (Hoozemans et al., 2005). In the current paper, Ferreiro et al. show induction of BiP levels, as well as decreased pro-caspase-12 levels induced by Aβ1-40. This may indicate that the ER stress response (including the apoptotic branch of the UPR) is activated directly by Aβ, and may be the cause of the UPR activation that we observe in AD neurons. However, the data obtained by Ferreiro et al. in vitro appear not to corroborate fully with observations from the actual patient material. The data presented in the Ferreiro paper suggest that apoptotic cell death is a direct consequence of Aβ-induced UPR activation, whereas we find no evidence of apoptosis in AD neurons with an activated UPR. The UPR is activated as a protective mechanism to restore ER homeostasis, and although it can result in cell death after prolonged activation, it is not necessarily a bad thing. This is in agreement with our observation that the UPR is activated relatively early in AD pathology. In this respect it would be interesting to distinguish effects of Aβ aggregation state (here only a fibrillar preparation of Aβ1-40 was used). Therefore, this paper adds to the emerging idea that the ER and the ER stress response are involved in AD pathogenesis, but caution is warranted to directly translate these in vitro data to the disease mechanism.

References:
Hoozemans JJ, Veerhuis R, Van Haastert ES, Rozemuller JM, Baas F, Eikelenboom P, Scheper W. The unfolded protein response is activated in Alzheimer's disease. Acta Neuropathol (Berl). 2005 Aug;110(2):165-72. Epub 2005 Jun 23. Abstract

Scheper W, Hol EM. Protein quality control in Alzheimer's disease: a fatal saviour. Curr Drug Targets CNS Neurol Disord. 2005 Jun;4(3):283-92. Review. Abstract

View all comments by Jeroen Hoozemans
View all comments by Wiep Scheper

The research community appears to play with half a deck of cards by ignoring the role of metals, particularly aluminum in co-causation of Alzheimer dementia. Ghribi et al., in a series of studies, investigated the effect of aluminum on the endoplasmic reticulum and mitochondria, and reported that the metal caused apoptosis through changes in cytochrome c, Bcl-2 and Bax in the hippocampus of aluminum-treated rabbits. There is cross-talk between the metal and amyloid, as the two toxins bond to each other, and the metal affects processing of amyloid. The aging brain has bio-accumulated a substantial amount of aluminum by age 60. Must we now move beyond a one-dimensional view of AD to make progress? Most chronic diseases of the aging process have multiple causation.

References:
Ghribi O, DeWitt DA, Forbes MS, Herman MM, Savory J. Co-involvement of mitochondria and endoplasmic reticulum in regulation of apoptosis: changes in cytochrome c, Bcl-2 and Bax in the hippocampus of aluminum-treated rabbits. Brain Res. 2001 Jun 8;903(1-2):66-73. Abstract

View all comments by Erik Jansson

In a recent review paper (Ghribi, 2006), we have addressed the role of ER in Alzheimer disease and discussed data supporting dysfunction of the ER as an early event leading to Aβ accumulation in familial AD. We have also discussed the possible role of oxidative stress and other factors as contributors in Aβ accumulation by reducing the clearance of Aβ from the endoplasmic reticulum. Our previous work (Ghribi et al., 2004; 2003) also demonstrated ER stress as a mechanism underlying exogenous Aβ neurotoxicity.

References:
Ghribi O. The role of the endoplasmic reticulum in the accumulation of beta-amyloid peptide in Alzheimer's disease. Curr Mol Med. 2006;6(1):119-33. Review. Abstract

Ghribi O, Herman MM, Pramoonjago P, Spaulding NK, Savory J. GDNF regulates the A beta-induced endoplasmic reticulum stress response in rabbit hippocampus by inhibiting the activation of gadd 153 and the JNK and ERK kinases. Neurobiol Dis. 2004;16(2):417-27. Abstract

Ghribi O, Herman MM, Savory J. Lithium inhibits Abeta-induced stress in endoplasmic reticulum of rabbit hippocampus but does not prevent oxidative damage and tau phosphorylation. J Neurosci Res. 2003;71(6):853-62. Abstract

View all comments by Othman Ghribi

This paper shows the involvement of calcium released from the endoplasmic reticulum (ER) in neuronal death induced by a synthetic prion peptide and by the Aβ peptide as causative agents in prion and Alzheimer diseases, respectively. The work is done using cultured cortical neurons and demonstrates a cascade of events causing neuronal demise. This pathway is triggered by elevated calcium that can be blocked by inhibition of ER calcium channels.

Calcium dysregulations have long been considered as a part of neuronal toxicity in AD, as also shown by mutations in presenilins. Likewise, infected cells in prion disease show calcium elevation but the mechanisms causing cell death have remained elusive. This paper shows a possible mechanism by which disturbed calcium regulation causes cell death through a crosstalk between the ER and mitochondria leading ultimately to caspase activation. The paper is highly recommended.

View all comments by Dan Lindholm

This paper from Saxena et al. is a very interesting, even outstanding paper. Despite that ER stress has been conceptually linked before to ALS development, the experiments performed here offer a novel view on the chronology of facts before denervation and symptom development in relevant experimental models. It should be useful also for other diseases, where ER stress has been also involved.

Several findings are really surprising: 1) the clear division between resistant motor neurons (RES) and vulnerable ones (VUL); 2) the predictability on development of the disease that the pathogenic scheme described by authors allows; 3) the dissociation between ubiquitination—often considered a pathological hallmark for this disease and other neurodegenerative diseases—and real axonal pathology; 4) the very early changes at a cellular level (as early as postnatal 5 in some markers) that preclude pathological changes; 5) the presence of novel markers of the disease at an immunological level (such as ATF3, PERK, and similar); 6) the distinctive patterns of expression between RES and VUL...  Read more

This paper from Saxena et al. is a very interesting, even outstanding paper. Despite that ER stress has been conceptually linked before to ALS development, the experiments performed here offer a novel view on the chronology of facts before denervation and symptom development in relevant experimental models. It should be useful also for other diseases, where ER stress has been also involved.

Several findings are really surprising: 1) the clear division between resistant motor neurons (RES) and vulnerable ones (VUL); 2) the predictability on development of the disease that the pathogenic scheme described by authors allows; 3) the dissociation between ubiquitination—often considered a pathological hallmark for this disease and other neurodegenerative diseases—and real axonal pathology; 4) the very early changes at a cellular level (as early as postnatal 5 in some markers) that preclude pathological changes; 5) the presence of novel markers of the disease at an immunological level (such as ATF3, PERK, and similar); 6) the distinctive patterns of expression between RES and VUL neurons; 7) the interplay between growth factor treatment (CTNF) and rescue in ER stress terms; and 8) the positive effect of salubrinal in ALS development and the negative effects of crushing schemes

The findings fit quite well with some of the "usual suspects" theories of ALS, such as the involvement of mitochondria and glia. Mitochondrial impairment (due to unfolded SOD or to other events) would lead to lower ATP levels or to Ca homeostasis dysregulation, which would affect the ER, increasing the unfolded protein response (UPR). Additionally, and pertinent to our case since we linked ER stress to oxidative stress, ER folding capacities are strongly influenced by oxidative milieu and the findings reported here (including participation of hypoxia and NRf2 dependent pathways) agree with the potentially increased oxidative stress in VUL neurons. Most interestingly, many data (as recently reviewed by Cleveland et al. in the last Cell volume—see Lagier-Tourenne et al., 2009) point out the importance of RNA processing in ALS. The hypothetical interplay between alterations in RNA splicing and ER stress in VUL neurons is in agreement with the high structural and energetical requirements of those cells. It seems that long axons and the structural and functional needs that those "near to pathology" cells (VUL motor neurons) exhibit, make them extremely prone to pathology.

It is also somewhat surprising that ER stress, which may be considered a logical and physiological consequence of UPR, is followed by cellular demise. It would be also be very interesting, as apoptosis seems not involved, to characterize the distal events of ER stress; i.e., what is the link between ER stress and denervation. This is because although the authors define that salubrinal treatment is useful at preclinical stages, it seems that its efficiency would be much lower at a clinical stage.

It would be nice to confirm these results in human samples. Though it could be difficult to find VUL and RES motor neurons in samples from human disease specimens, but this would also be useful to extend those findings to the more common form of the disease (sporadic ALS, by far, is commoner than the familial form). Further experiments would have to prove, by in vitro transfection with some of the factors described in the papers, that RESistance to disease is acquired by VULnerable neurons (or vice versa, by using RNAi or similar techniques).

View all comments by Manuel Portero

This paper from Eckhart Mandelkow's group seems directly related to the question at hand:

Ebneth A, Godemann R, Stamer K, Illenberger S, Trinczek B, Mandelkow E. Overexpression of tau protein inhibits kinesin-dependent trafficking of vesicles, mitochondria, and endoplasmic reticulum: implications for Alzheimer's disease. J Cell Biol. 1998 Nov 2;143(3):777-94. Abstract

View all comments by P.F. Jennings

The article is interesting and paves the way to new insights on the pathogenic mechanisms of SOD-linked ALS.

The data that are most convincing for us are the studies in transgenic mice: there is a clear correlation between the expression levels of Nogo-A and mouse survival and motor ability; this underlines the critical role of Nogo-A in protecting neurons from SOD1-dependent toxicity.

The PDI redistribution upon Nogo-A overexpression is also interesting, but we think that the pathway that leads to this effect is not clear. Is it mediated by a direct interaction or are other proteins involved? What is the biological significance of PDI puncta within the cell?

The main problem for us is that the link between PDI redistribution and the protective role of Nogo-A in ALS is purely correlative. Although it is possible that Nogo-A protects motor neurons by redistributing PDI, this has not been demonstrated. We would like to know more on how redistributed PDI can prevent motor neuron degeneration.

View all comments by Elisa Fasana
View all comments by Matteo Fossati

Since 2002, Nogo isoforms have been suggested as potentially useful biomarkers for ALS diagnosis and prognosis. Recent findings have indicated that disease severity may be correlated with Nogo isoform expression levels in the muscles, although this phenomenon may not be specific for ALS, and occurs also in other forms of myopathies.

Nogo-A’s role in ALS is not clearly understood. Is it just a bystander, does it play a role in aggravating the disease, or does it actually help protect against ALS? A previous report (Jokic et al., 2006) has suggested that Nogo-A may be a causative factor or has a role in disease progression, as the authors found that Nogo-A knockout could increase the survival period of ALS SOD(G86R) mice, while its overexpression destabilized neuromuscular junctions, which would eventually result in motor neuron death.

The paper by Yang et al. (2009) provides a contrasting and interesting role for Nogo-A in ALS. The authors showed that Nogo-A may function to enhance survival in ALS mice by redistributing the endoplasmic reticulum (ER) chaperone, protein...  Read more

Since 2002, Nogo isoforms have been suggested as potentially useful biomarkers for ALS diagnosis and prognosis. Recent findings have indicated that disease severity may be correlated with Nogo isoform expression levels in the muscles, although this phenomenon may not be specific for ALS, and occurs also in other forms of myopathies.

Nogo-A’s role in ALS is not clearly understood. Is it just a bystander, does it play a role in aggravating the disease, or does it actually help protect against ALS? A previous report (Jokic et al., 2006) has suggested that Nogo-A may be a causative factor or has a role in disease progression, as the authors found that Nogo-A knockout could increase the survival period of ALS SOD(G86R) mice, while its overexpression destabilized neuromuscular junctions, which would eventually result in motor neuron death.

The paper by Yang et al. (2009) provides a contrasting and interesting role for Nogo-A in ALS. The authors showed that Nogo-A may function to enhance survival in ALS mice by redistributing the endoplasmic reticulum (ER) chaperone, protein disulfide isomerise (PDI), to a subcellular compartment of uncertain identity. In contrast to the earlier report, Yang et al. found that deletion of Nogo-A/B accelerated axonal degeneration of another ALS mutant SOD model, the SOD(G93A) mice. Walker’s commentary (2010) on the Yang paper is very comprehensive, and the author has made several cogent and insightful comments on several aspects of the Yang paper.

In furthering the discussion, I feel that there are two important and interesting aspects to the Yang paper that warrant further investigation by workers in the field. Firstly, from a cell biological perspective, it would be exciting to find out exactly to which subcellular compartment PDI is redistributed. Based on a rather limited marker profile, the authors have ruled out Golgi, endosomes, and vesicles in the autophagy pathway. I doubt that the spots are simply protein aggregates. One possibility is that PDI has been redistributed to specific parts of the ER, such as the ER exit sites or the ER-Golgi intermediate compartment (ERGIC) (Appenzeller-Herzog and Hauri, 2006). PDI has been shown to be functionally inactivated in ALS by S-nitrosylation and as such could no longer be protective against misfolded proteins in disease conditions (Walker et al., 2010). Therefore, could Nogo-A aid in the removal of dysfunctional PDI from the ER, and in doing so, eventually enhance protein folding in ER and hence survival? This, of course, begs the question of how Nogo-A could affect PDI’s redistribution without directly interacting or colocalizing with the latter. It should be noted that all Nogo isoforms are primarily ER residents, and Nogo-A and B have been implicated to act in the modulation of ER morphology and shape (Voeltz et al., 2006). Therefore, Nogo isoform expression levels are likely to influence the dynamics and distribution of ER residents, such as the KDEL signal-containing proteins. How this influence is connected to pathological conditions like ALS should be an interesting line of investigation.

The second, more clinically relevant point is the contrasting results between the studies by Jokic et al. and Yang et al. It would be interesting, if only on a speculative basis, to try to understand how such a discrepancy could arise. There are two notable differences between the mouse models used by the different group of authors. Firstly, the nature of the SOD1 mutation is different (G86R for Jokic et al. and G93A for Yang et al.). Secondly, and perhaps connected to a controversy in the Nogo field (Teng and Tang, 2005), the two studies differ in the Nogo knockout mice used. The model used by Jokic et al. is based on the Nogo knockout generated by Simonen et al. (2003), with parts of nogo exons 2 and 3 and the intron between them deleted. The model used by Yang et al., on the other hand, is based on one reported by Kim et al. (2003), generated by a gene trap insertion that maps near the 5′ end of exon 3. The mice used by Simonen et al. no longer expressed the Nogo-A isoform, but both Nogo-B and Nogo-C, the other two major Nogo isoforms, remained expressed. In fact, there appears to be a compensatory upregulation of Nogo-B in the CNS of the mice used by Simonen et al. The mice used by Kim et al., on the other hand, have both Nogo-A and Nogo-B isoforms deleted. In the initial reports on effects of the respective knockouts on axonal regeneration, the mice used by Kim et al. appeared to have a better enhancement in regenerative capacity. The differences in the nature of SOD1 mutant and Nogo isoform deletion could potentially contribute towards the contrasting conclusions reached by the different authors on the role of Nogo in ALS disease onset and progression. For reasons yet unclear, the Jokic/Simonen mice were less prone to mutant SOD1-induced ALS motor neuron degeneration compared to wild-type control, while the Yang/Kim mice were more disease susceptible compared to wild-type. Further comparative investigations would shed light on the differences between these animals.

Many questions still loom ahead with regard to Nogo-A’s actual role and importance in ALS. Efforts in resolving these questions could make crucial contributions toward the prevention and treatment of the disease.

References:
Appenzeller-Herzog C, Hauri HP (2006) The ER-Golgi intermediate compartment (ERGIC): in search of its identity and function. J Cell Sci. 119: 2173-2183. Abstract

Jokic N, Gonzalez de Aguilar JL, Dimou L, Lin S, Fergani A, Ruegg MA, Schwab ME, Dupuis L, Loeffler JP (2006) The neurite outgrowth inhibitor Nogo-A promotes denervation in an amyotrophic lateral sclerosis model. EMBO reports 7:1162–1167. Abstract

Kim JE, Li S, GrandPré T, Qiu D, Strittmatter SM (2003) Axon regeneration in young adult mice lacking Nogo-A/B. Neuron 38:187-199. Abstract

Simonen M, Pedersen V, Weinmann O, Schnell L, Buss A, Ledermann B, Christ F, Sansig G, van der Putten H, Schwab ME (2003) Systemic deletion of the myelin-associated outgrowth inhibitor Nogo-A improves regenerative and plastic responses after spinal cord injury. Neuron 38: 201-211. Abstract

Teng FY and Tang BL (2005) Why do Nogo/Nogo-66 receptor gene knockouts result in inferior regeneration compared to treatment with neutralizing agents? J Neurochem. 94: 865-874. Abstract

Walker AK (2010) Protein disulfide isomerase and the endoplasmic reticulum in amyotrophic lateral sclerosis. J Neurosci. 30: 3865-3867. Abstract

Walker AK, Farg MA, Bye CR, McLean CA, Horne MK and Atkin JD (2010) Protein disulphide isomerase protects against protein aggregation and is S-nitrosylated in amyotrophic lateral sclerosis. Brain 133: 105-116. Abstract

Yang YS, Harel NY, Strittmatter SM (2009) Reticulon-4A (Nogo-A) redistributes protein disulfide isomerase to protect mice from SOD1-dependent amyotrophic lateral sclerosis. J Neurosci. 29: 13850-13859. Abstract

View all comments by Felicia Y.T. Teng