The Journal of Neuroscience, March 17, 2010 • 30(11):3865–3867 • 3865

Journal Club

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Protein Disulfide Isomerase and the in Amyotrophic Lateral Sclerosis

Adam K. Walker1,2 1Howard Florey Institute, Florey Neuroscience Institutes, and Centre for Neuroscience, The University of Melbourne, Parkville, Victoria 3010, Australia, and 2Department of Biochemistry, La Trobe University, Bundoora, Victoria 3086, Australia Review of Yang et al.

Amyotrophic lateral sclerosis (ALS) is a continue to accumulate. The vitro. Furthermore, the authors showed devastating neurodegenerative disease UPR is activated in motor neurons of the that deletion of the gene encoding reti- primarily affecting motor neurons. No most commonly used ALS model, high- culon-4A,B (Rtn-4A,B, also known as previous family history exists in the ma- expressing mutant SOD1 G93A transgenic NogoA,B) prevents PDI redistribution and jority of cases, but mutations in the gene mice, as early as postnatal day 5, long be- accelerates degeneration and disease pro- encoding superoxide dismutase 1 (SOD1) fore symptom onset at approximately day gression in SOD1 G93A mice. These findings cause ϳ20% of familial disease. Although 90 (Saxena et al., 2009). Additionally, ge- suggest that reticulon expression and PDI the mechanisms causing motor neuron netic ablation of the ER stress-activated localization could be important modulating degeneration in ALS remain unknown, transcription factor X-box-binding -1 factors in ALS. recent findings highlight a critical role for significantly delays disease onset and in- Initially, the authors identified that the endoplasmic reticulum (ER) in dis- creases survival in mutant SOD1 mice (Hetz Rtn-4A expression altered the distribu- ease pathogenesis. Protein misfolding in et al., 2009). These results indicate that ER tion of proteins containing a lysine–aspar- the ER lumen can activate signaling trans- stress could be an early event in the patho- tic acid–glutamic acid–leucine (KDEL) duction pathways known collectively as genic cascade in ALS. motif, which is present at the far C termi- the unfolded protein response (UPR), One important UPR-induced chaper- nus of many ER-localized proteins to which occurs in mutant SOD1 models of one is protein disulfide isomerase (PDI). allow receptor-mediated ER retention. ALS (Atkin et al., 2006), and in PDI is an abundant that forms, Rtn-4A expression in simian fibroblast sporadic disease (Atkin et al., 2008). breaks, and isomerizes disulfide bonds COS-7 cells, which normally have no de- The UPR attempts to resolve ER stress and is therefore an important cellular de- tectable Rtn-4A, redistributed KDEL pro- by inhibiting protein translation, increas- fense against protein misfolding. PDI is teins from a reticular pattern, reminiscent ing production of chaperone proteins, upregulated before symptom onset in spi- of the normal ER, to a more punctate pat- and enhancing protein degradation; how- nal cords of mutant SOD1 G93A mice and tern (Yang et al., 2009, their Fig. 1). Using ever, death is triggered if misfolded rats and in human ALS postmortem tissue directed against individual (Atkin et al., 2008). Although PDI can be KDEL proteins, the authors showed that protective against mutant SOD1 aggrega- Rtn-4A expression changed PDI distribu- Received Jan. 25, 2010; revised Feb. 4, 2010; accepted Feb. 5, 2010. tion and toxicity, aberrant S-nitrosylation tion to a high degree, whereas other KDEL A.K.W. is supported by an Australian Postgraduate Award and an Aus- tralian Rotary Health scholarship. I thank Dr. Julie Atkin and Prof. Malcolm of critical active site cysteine residues likely proteins, such as Grp94 (glucose-regu- Horne for helpful comments on this manuscript. Related studies in the inactivates the normal protective function lated protein 94), calreticulin, and BiP (Ig Atkin and Horne laboratories were supported in part by National Health of PDI in ALS spinal cords (Walker et al., binding protein), displayed little or no and Medical Research Council of Australia Project Grants 454749 and 2010). change in localization (Yang et al., 2009, 236805, Amyotrophic Lateral Sclerosis Association (United States), Motor Neurone Disease Research Institute of Australia, Bethlehem Griffiths Re- In a recent paper published in The their Fig. 2). Furthermore, expression of search Foundation, and a Henry H. Roth Charitable Foundation Grant for Journal of Neuroscience, Yang et al. (2009) different isoforms of the four mammalian Motor Neurone Disease Research. further investigated the role of PDI in ALS reticulon proteins, namely Rtn-1C, Rtn- Correspondence should be addressed to Adam K. Walker, Florey Neuro- and identified reticulons, a family of inte- 2A, Rtn-3, and Rtn-4C (NogoC), also science Institutes, The University of Melbourne, Parkville, Victoria 3010, Australia. E-mail: [email protected]. gral ER membrane proteins of primarily caused PDI redistribution (Yang et al., DOI:10.1523/JNEUROSCI.0408-10.2010 ill-defined function, as important modu- 2009, their Fig. 3). Therefore, redistribu- Copyright © 2010 the authors 0270-6474/10/303865-03$15.00/0 lators of PDI subcellular localization in tion of PDI to ER subcompartments or 3866 • J. Neurosci., March 17, 2010 • 30(11):3865–3867 Walker • Journal Club other regions within the cell could be a lons and microtubules, could be impor- reticulons in ER stress signaling, whether previously unidentified function of the re- tant for motor neuron survival. neutral, protective, or detrimental, thus re- ticulon protein family. Yang et al. (2009) further confirmed mains uncertain (Teng and Tang, 2008). How do reticulons mediate PDI re- that reticulons cause PDI redistribution in Another unanswered question is whether distribution? Little colocalization was mouse motor neurons in vivo. Genetic de- reticulon-mediated redistribution is spe- observed between PDI and reticulon pro- letion of Rtn-4A,B resulted in a more ho- cific for PDI or involves other ER proteins teins, indicating that changes in PDI lo- mogenous distribution of PDI with fewer not investigated in this study. The authors calization were not mediated by direct PDI puncta in spinal motor neurons com- note that expression of both Rtn-1C and interaction (Yang et al., 2009, their Fig. 3). pared with those of wild-type mice, with- Rtn-4C caused redistribution of the PDI Reticulon involvement in microtubule- out any change in distribution of the ER family member ERp57, albeit to a lesser based intracellular and axonal transport transmembrane protein calnexin (Yang et extent than PDI itself (Yang et al., 2009, processes implicated in many adult-onset al., 2009, their Fig. 5). Conversely, Rtn-4A their supplemental Fig. 1, available at neurodegenerative diseases, including ALS, overexpression in motor neurons increased www.jneurosci.org as supplemental ma- could be one explanation (Morfini et al., PDI puncta formation, again without terial). Upregulation of ERp57 was found 2009). change in calnexin distribution (Yang et al., previously in both SOD1 G93A mice and Interestingly, Rtn-3 is both antero- 2009, their Fig. 5). rats, indicating that other PDI family gradely and retrogradely transported in To demonstrate that reticulon expres- members are involved in disease (Atkin et , and overexpression of Rtn-3 causes sion affected ALS disease onset and pro- al., 2006). It is therefore important to an imbalance in axonal cargo transport gression, the authors showed that genetic identify which other ER proteins show al- (Shi et al., 2009). Whether other reticu- deletion of Rtn-4A,B accelerated axonal tered distribution in the reticulon dele- lons are also involved in microtubule- degeneration in spinal cords of transgenic tion and overexpression mice. based transport remains to be seen, but SOD1 G93A-expressing mice (Yang et al., Finally, an issue not directly assessed in mutation of the microtubule-associated 2009, their Fig. 6). Additionally, SOD1 G93A this study is the functional consequences protein mini spindles in alters mice with deletion of one or both Rtn-4A,B of PDI redistribution. PDI is highly ex- localization of both PDI and the Dro- alleles reached disease end-stage signifi- pressed in spinal cord motor neurons sophila reticulon protein reticulon like-1 cantly earlier than controls, with a decrease (Atkin et al., 2006) and undergoes aber- (Pokrywka et al., 2009). This suggests that in survival of up to 13 d in Rtn-4A,Bϩ/Ϫ rant posttranslational modification in reticulons affect microtubule-based ax- mice and 20 d in Rtn-4A,BϪ/Ϫ mice (Yang et ALS (Walker et al., 2010), indicating that onal transport and that distribution of al., 2009, their Fig. 8). Motor performance PDI function could affect motor neuron both PDI and reticulons depends on mi- was also decreased and weight loss was ac- survival. Reticulon-mediated PDI redis- crotubule-based transport. celerated in SOD1 G93A mice with Rtn-4A,B tribution could lead to an increase in A second, although not mutually ex- deletion. These results suggest that Rtn- PDI activity as a result of altered protein clusive, explanation for the altered PDI 4A,B protects against SOD1 G93A-mediated interactions or increased availability of distribution could be the effect of reticu- neuronal degeneration, although future substrates. SOD1 could be one of the sub- lons on ER morphology. Rtn-3 expression demonstration of a beneficial effect by re- strates affected by PDI redistribution, and prevents the disruption of ER structure ticulon overexpression in SOD1 G93A mice is it would be interesting to investigate normally seen after nocodozole-induced important to confirm this finding. whether mutant SOD1 aggregation was microtubule depolymerization (Shibata Whether reticulon-mediated PDI re- exacerbated by Rtn-4A,B deletion in the et al., 2008). Furthermore, ER morphol- distribution was the sole factor modulat- SOD1 G93A mice. It is possible that the in- ogy defects have been proposed as a ing disease course in SOD1 G93A mice with teraction of PDI with other substrate pro- pathogenic effect of atlastin-1 mutations, Rtn-4A,B ablation remains unclear. An- teins not yet identified could also be which cause a subset of hereditary spastic other possibility is that reticulon deletion involved in disease. paraplegia, another disease affecting mo- affected ER stress signaling, which indi- In conclusion, Yang et al. (2009) pro- tor neurons (Hu et al., 2009). In addition rectly affected survival. The au- vide strong evidence for a role of reticulon to interacting with Rtn-4A, atlastin-1 thors reported that the UPR was not proteins in modulating mutant SOD1- interacts with the microtubule-severing altered with overexpression of Rtn-1C, linked ALS, possibly by altering PDI sub- protein spastin, and spastin mutations Rtn-2A, Rtn-3, Rtn-4A, or Rtn-4C in cellular distribution. Additional work is cause another subset of hereditary spastic COS-7 cells (Yang et al., 2009, their Fig. needed to define the microtubule involve- paraplegia (Evans et al., 2006). Overall, 4). However, UPR induction was seen ment in changes to ER structure, which these findings indicate that microtubule- previously in neuroblastoma cells but not could link reticulon function with trans- based transport and ER structural changes in COS-7 cells, with mutant SOD1 expres- port failures in ALS. Whether ER stress are linked to both reticulons and other sion (Atkin et al., 2006). Therefore, it signaling, which is an early event in ALS, is proteins involved in diseases affecting would be pertinent to investigate the ef- involved in reticulon-mediated motor motor neurons. Indeed, the ALS-linked fect of reticulons on the UPR in neuronal neuron protection also remains unclear. mutant VAPB (vesicle-associated mem- cell culture and in the reticulon overex- Nonetheless, this study identifies reticu- brane protein-associated protein B), a pressing and deletion mouse lines used in lons and reticulon-mediated ER changes, microtubule-associated protein, induces this study. Importantly, previous studies including PDI redistribution, as potential formation of inclusions characterized as a showed that Rtn-1C overexpression in- therapeutic targets in ALS. form of restructured organized smooth creased susceptibility to ER stress in neu- ER, containing a subset of ER-derived roblastoma cells (Di Sano et al., 2007), References proteins (Fasana et al., 2010). A pathway whereas ER stress-induced Rtn-3 expres- Atkin JD, Farg MA, Turner BJ, Tomas D, Lysaght involving changes to ER structure and sion can be protective against various in- JA, Nunan J, Rembach A, Nagley P, Beart PM, protein distribution, mediated by reticu- sults (Wan et al., 2007). The role of Cheema SS, Horne MK (2006) Induction of Walker • Journal Club J. Neurosci., March 17, 2010 • 30(11):3865–3867 • 3867

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