Squash an axon and the whole neuron dies. Researchers do not fully understand how the loss of that cellular limb causes the relatively undamaged cell body to degenerate. A paper in the February 9 Neuron places blame on the endoplasmic reticulum’s (ER’s) unfolded protein response (UPR), even though misfolded peptides have nothing to do with the attempted triage. In this case, at least, unfolded protein response would be a misnomer. The authors, from Harvard Medical School, also report that boosting an alternative ER response keeps neurons alive after axonal damage. If a treatment could be found that balances these deleterious and beneficial pathways, the authors suggest, it could preserve neurons in the face of injury or disease, and provide a first step toward axon regeneration.

The UPR turns on early during neurodegenerative conditions, including amyotrophic lateral sclerosis, Parkinson’s, Alzheimer’s, and other tauopathies (Atkin et al., 2008; Hoozemans et al., 2012; Nijholt et al., 2011; Ferreiro and Pereira, 2011). “The lessons learned through these axonal damage studies might have implications beyond injury-related cell death and neural repair,” noted Francesco Roselli and Pico Caroni of the Friedrich Meischer Institute in Basel, Switzerland, in a commentary accompanying the Neuron paper.

Study author Yang Hu, now at Temple University in Philadelphia, set out to understand the consequences of ER stress pathways after injury. Hu started the work in the Harvard lab of Zhigang He, co-senior author with Dong Feng Chen. Hu, a former ophthalmologist, chose to work with retinal ganglion cells because of their clear, straightforward anatomy. Together with co-first authors Liu Yang, Kevin Park, and Xin Wei in the Chen lab, Hu crushed the axons and purified the cell bodies to examine the expression of ER stress markers.

Upon ER stress, three biochemical pathways awaken, leading to the activation of three transcription factors: CHOP (CCAAT/enhancer binding homologous protein), XBP-1, and ATF6 (activating transcription factor 6). The precise downstream effects of these factors are far from straightforward. XBP-1 acts on multiple gene targets, including those responsible for ER membrane synthesis, protein-folding chaperones, and destroying misfolded peptides (Sriburi et al., 2004; Lee et al., 2003). Yet XBP-1 is not always beneficial; for example, it contributes to ALS-like symptoms in mice (see ARF related news story on Hetz et al., 2009). CHOP’s effects include starting up apoptosis, but it can also protect neurons (Oyadomari and Mori, 2004; Halterman et al., 2010). The sum of the three UPR pathways “can be anti- or pro-apoptotic depending on the trigger, intensity, and cellular context of UPR activation,” Roselli and Caroni wrote (Han et al., 2009).

Hu and colleagues focused on the activities of XBP-1 and CHOP, which have been studied more than ATF6. Since the ER stress pathways are thought to work together (Ron and Walter, 2007), Hu was surprised to find that the axon crush strongly upregulated CHOP for at least a week, but XBP-1 only slightly and transiently.

To examine what the two proteins do following axon injury, the researchers repeated the axon crush experiments in CHOP knockout mice or conditional knockouts missing XBP-1 in the retinal ganglia, and then counted surviving retinal neurons in the optic nerve. Getting rid of CHOP was beneficial: Fifty-two percent of the neurons lived past axon crush, while only 24 percent survived in wild-type controls. However, XBP-1 knockout did not protect the neurons. Surprised again, Hu said, “We thought, this molecule may have the opposite effect of CHOP.” When the researchers overexpressed XBP-1, 64 percent of neurons survived axon damage versus 20 percent in the control animals. “Taken together, these results suggest that XBP-1 and CHOP play opposite roles in controlling neuronal survival after axonal injury,” wrote the authors.

Hu repeated the experiments in a mouse model of glaucoma. This eye disease results from increased pressure inside the eye; the pressure squeezes the axons that reach into the eye’s fluid-filled cavity. Hu and colleagues injected beads into the eye’s anterior chamber to block fluid draining, boosting the internal pressure. As with the acute crush injury, the pseudo-glaucoma turned on CHOP but only a bit of XBP-1, and neurons degenerated. Adding XBP-1 or limiting CHOP rescued them. The XBP-1 treatment was effective even a week after the bead injection. This last scenario is more relevant to people, who would be treated after damage had occurred, noted Lawrence Wrabetz of the State University of New York at Buffalo in an e-mail to ARF. “This paper provides further support for the idea that ER stress responses are a mixed bag—some beneficial and some detrimental,” he added (Gow and Wrabetz, 2009).

Despite the UPR pathway, there is no evidence that misfolded proteins fill the retinal neurons upon axon injury, Hu noted in an e-mail to ARF. Thus, he speculated, the UPR activated in Alzheimer’s and other conditions might not be the result of misfolded proteins, but of axonal injury. “The results of Hu et al. suggest that, in neurons, a UPR can be an intrinsic response to disturbances in axonal integrity and flow, possibly unrelated to the load of un/misfolded proteins,” concurred Roselli and Caroni. This supports the idea that the UPR is not wholly involved in managing un- or misfolded proteins, but responds to a variety of cell stresses and demands (see review by Rutkowski and Hegde, 2010). Therapeutics promoting XBP-1 activity might be applicable in a number of neurodegenerative diseases, Wrabetz suggested.

Plenty of questions remain about these pathways. “We still do not know how XBP-1 protects these injured neurons,” Hu wrote in an e-mail. Also by e-mail, He speculated it might upregulate chaperone proteins. Keeping the cell bodies alive is only half the battle; they will need axons to function. It is not yet clear, Hu said, if adding XBP-1 or removing CHOP assists axons as well. “Whether this also can preserve the axon is still the question we need to answer,” he said.—Amber Dance

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  1. In this paper, Hu and colleagues explore the role of the unfolded protein response (UPR) in neurodegeneration of retinal ganglion cells in the context of traumatic optic nerve injury and glaucoma. The data presented may indicate that selective activation of one of the three UPR signaling pathways may occur in neurons.

    The authors raise the interesting idea that the morphological structure of the neuron contributes to a specific regulation of the UPR that differs from other cell types. The signaling cascade initiated by the UPR might be different in the axons compared to that in the soma of the neuron. This is an interesting aspect; however, this requires further investigation using more specific UPR activation markers because non-UPR signaling pathways converge with the UPR on the downstream targets analyzed in this study.

    Hu et al. show that differential modulation of targets located in different signaling cascades of the UPR stimulates neuronal survival. In their models, they show that deletion of CHOP and increased expression of XBP-1 improve neuronal survival. However, in the current study, no link was observed between these effects and the activation of regenerative pathways. Somehow this makes sense, because during endoplasmic reticulum (ER) stress a cell’s first priority would be restoration of ER homeostasis before embarking on cell division and other regenerative processes. Activation of the UPR in neuroblastoma cells induces a cell cycle arrest, and in AD the UPR activation markers negatively correlate with the expression of cell cycle proteins, which potentially have a regenerative function in early AD (1). Signaling cascades of the UPR that actively block a regenerative process could play an important role in neurodegenerative diseases, and need to be addressed in future studies.

    In neurodegenerative diseases like Alzheimer’s disease (AD), frontotemporal dementia (FTD), and Parkinson’s disease, UPR activation is observed in neurons in close association with the accumulation of misfolded proteins that occurs in these diseases (2-4). In contrast to the observations in the traumatic optic nerve injury models, the available data support activation of all signaling pathways of the UPR in neurodegenerative diseases. Although the UPR is initially protective, it is suggested that its prolonged activation contributes to the neurodegenerative process. However, the outcome of UPR activation due to acute and chronic stress appears different. For instance, UPR activation after traumatic optic nerve injury contributes to neuronal loss via apoptosis, while there is substantial evidence that in AD and FTD, the UPR is involved in the phosphorylation and accumulation of the microtubule-associated protein, tau. The involvement of the UPR in these different diseases suggests that the UPR could be addressed as a therapeutic target to reduce neurodegenerative processes in general. Therefore, more insight should be gained in how the different signaling cascades of the UPR contribute to neurodegeneration. A part of this issue is nicely addressed by Hu and colleagues in the current paper.

    References:

    . The unfolded protein response affects neuronal cell cycle protein expression: implications for Alzheimer's disease pathogenesis. Exp Gerontol. 2006 Apr;41(4):380-6. PubMed.

    . Activation of the unfolded protein response in Parkinson's disease. Biochem Biophys Res Commun. 2007 Mar 16;354(3):707-11. PubMed.

    . The unfolded protein response is activated in pretangle neurons in Alzheimer's disease hippocampus. Am J Pathol. 2009 Apr;174(4):1241-51. PubMed.

    . The unfolded protein response is associated with early tau pathology in the hippocampus of tauopathies. J Pathol. 2011 Nov 21; PubMed.

    View all comments by Wiep Scheper

References

News Citations

  1. Research Brief: Mutant Cells Eat Mutant SOD1

Paper Citations

  1. . Endoplasmic reticulum stress and induction of the unfolded protein response in human sporadic amyotrophic lateral sclerosis. Neurobiol Dis. 2008 Jun;30(3):400-7. PubMed.
  2. . Activation of the unfolded protein response is an early event in Alzheimer's and Parkinson's disease. Neurodegener Dis. 2012;10(1-4):212-5. PubMed.
  3. . The unfolded protein response is associated with early tau pathology in the hippocampus of tauopathies. J Pathol. 2011 Nov 21; PubMed.
  4. . Endoplasmic reticulum stress: a new playER in tauopathies. J Pathol. 2012 Apr;226(5):687-92. PubMed.
  5. . XBP1: a link between the unfolded protein response, lipid biosynthesis, and biogenesis of the endoplasmic reticulum. J Cell Biol. 2004 Oct 11;167(1):35-41. PubMed.
  6. . XBP-1 regulates a subset of endoplasmic reticulum resident chaperone genes in the unfolded protein response. Mol Cell Biol. 2003 Nov;23(21):7448-59. PubMed.
  7. . XBP-1 deficiency in the nervous system protects against amyotrophic lateral sclerosis by increasing autophagy. Genes Dev. 2009 Oct 1;23(19):2294-306. PubMed.
  8. . Roles of CHOP/GADD153 in endoplasmic reticulum stress. Cell Death Differ. 2004 Apr;11(4):381-9. PubMed.
  9. . The endoplasmic reticulum stress response factor CHOP-10 protects against hypoxia-induced neuronal death. J Biol Chem. 2010 Jul 9;285(28):21329-40. PubMed.
  10. . IRE1alpha kinase activation modes control alternate endoribonuclease outputs to determine divergent cell fates. Cell. 2009 Aug 7;138(3):562-75. PubMed.
  11. . Signal integration in the endoplasmic reticulum unfolded protein response. Nat Rev Mol Cell Biol. 2007 Jul;8(7):519-29. PubMed.
  12. . CHOP and the endoplasmic reticulum stress response in myelinating glia. Curr Opin Neurobiol. 2009 Oct;19(5):505-10. PubMed.
  13. . Regulation of basal cellular physiology by the homeostatic unfolded protein response. J Cell Biol. 2010 May 31;189(5):783-94. PubMed.

Further Reading

Papers

  1. . Endoplasmic Reticulum Stress Induces Tau Pathology and Forms a Vicious Cycle: Implication in Alzheimer's Disease Pathogenesis. J Alzheimers Dis. 2011 Nov 18; PubMed.
  2. . Stress signaling from the endoplasmic reticulum: A central player in the pathogenesis of amyotrophic lateral sclerosis. IUBMB Life. 2011 Aug 10; PubMed.
  3. . Unfolded proteins and endoplasmic reticulum stress in neurodegenerative disorders. J Cell Mol Med. 2011 Oct;15(10):2025-39. PubMed.
  4. . The Role of ER Stress-Induced Apoptosis in Neurodegeneration. Curr Alzheimer Res. 2012 Mar 1;9(3):373-87. PubMed.
  5. . The unfolded protein response and proteostasis in Alzheimer disease: preferential activation of autophagy by endoplasmic reticulum stress. Autophagy. 2011 Aug 1;7(8):910-1. PubMed.
  6. . Endoplasmic reticulum stress activates autophagy but not the proteasome in neuronal cells: implications for Alzheimer's disease. Cell Death Differ. 2011 Jun;18(6):1071-81. PubMed.

Primary Papers

  1. . Differential effects of unfolded protein response pathways on axon injury-induced death of retinal ganglion cells. Neuron. 2012 Feb 9;73(3):445-52. PubMed.
  2. . Life-or-Death Decisions upon Axonal Damage. Neuron. 2012 Feb 9;73(3):405-7. PubMed.