When it works properly, the endoplasmic reticulum (ER) constantly pumps out newly synthesized membrane proteins, perfectly folded and sorted to their proper cellular destination. At the same time, this organelle keeps cellular calcium levels in balance. But when the ER gets overloaded and stressed, the cell counters with the unfolded protein response (UPR), a pathway which results in slowed protein synthesis and enhanced chaperone production to clear the backlog. If the overload persists, as it does in many neurodegenerative diseases featuring continuous production of mutant, malshaped proteins, such as Alzheimer disease (AD), cells undergo an ER-dependent form of apoptosis (see ARF related news story).
But ER stress can also come from without, according to new work from Claudia Pereira and colleagues at the University of Coimbra in Portugal. In a paper published online July 14 in the Neurobiology of Disease, the researchers report that application of Aβ1-40 to cultured neurons causes ER stress via pathological release of intracellular calcium stores. Chronically elevated intracellular calcium then leads to oxidative stress and cytochrome c release from mitochondria, triggering caspase activation and cell death. Blocking calcium release by inhibiting ER calcium channels reverses all these effects of Aβ and rescues neurons. Their results show that ER stress, induced by Aβ added to cells, can cooperate with mitochondrial pathways to trigger cell death. The results may apply to other diseases, too, since they showed that a neurotoxic prion peptide had very similar effects.
In other news from the ER, a study from Malcolm Horne and colleagues at the University of Melbourne in Australia shows upregulation of the UPR in SOD mutant models of ALS, and suggests that increased chaperone levels may be neuroprotective. Lastly, some basic research on the UPR reminds us once again how elegantly evolution solves life and death problems like protecting ER function. Work from Jonathan Weissman’s lab at the University of California, San Francisco, reveals a third arm to the UPR—in addition to transcriptional and translational responses, the cell also initiates degradation of mRNAs that specifically code for ER-targeted proteins.
Studies on the role of ER stress in Alzheimer disease have focused mostly on the presenilin proteins (PS). FAD-causing PS mutations interfere with protein folding and sensitize cells to ER stress-induced cell death by downregulating the UPR (see ARF related news story). But there have been hints that the ER stress-induced apoptosis could be involved in Aβ toxicity. Work from Junying Yuan’s lab at Harvard University showed that neurons from caspase-12 knockout mice were resistant to ER stress-induced cell death, and also Aβ toxicity (see ARF related news story). Soluble amyloid oligomers perturb calcium homeostasis in neurons, which is another trigger of ER stress (De Muro et al., 2005).
For these reasons, Pereira’s group looked specifically for ER-mediated apoptosis in response to exogenous Aβ1-40 peptides in cultured cortical neurons. First author Elisabete Ferreiro and colleagues showed that Aβ treatment increased ER stress, as indicated by elevated protein levels of the chaperone Grp78 and caspase-12 activation. Aβ also caused a rapid (within 1 hour) and sustained (up to 48 hours) increase in intracellular calcium. The calcium was derived from ER stores, since its accumulation was blocked by inhibiting either of the two major ER calcium release channels, the ryanodine receptor (RyR) and the inositol trisphosphate receptor (IP3R), with dantrolene or xestospongin C, respectively.
High intracellular calcium can stress out mitochondria, too, and the researchers showed that Aβ caused oxidative stress and apoptosis via a mitochondrial pathway. They recorded elevated production of reactive oxygen species, cytochrome c release from mitochondria, caspase activation (including the executioner caspase, caspase-3), and cell death. All these effects were inhibited by danotrolene or xestospongin C. From this data, the authors conclude that Aβ causes significant, early release of intracellular calcium, ER stress, and activation of the mitochondrial apoptosis pathway. Their results raise the possibility that calcium release channel blockers might be useful to protect against neuron loss in AD and prion diseases.
The UPR and ER stress-induced apoptosis also figure in the death of motor neurons triggered by mutant superoxide dismutase in ALS, according to the Australian researchers. In their paper, published online July 17 in the JBC, first author Julie Atkin and coworkers show that SOD1 mutant mice upregulate several markers of the UPR, including cleaved caspases-12, -9, and -3. They also found that the ER chaperone protein disulfide isomerase (PDI) was upregulated and associated with mutant SOD1 in rodent ALS models and in cells. Inhibiting PDI increased SOD1 aggregation, suggesting that the increased PDI they observed might represent a neuroprotective response. This report jibes with a paper earlier this year from Stuart Lipton, Eliezer Masliah, and Yasuyuki Normura describing inactivation of PDI in brains of AD and PD patients and suggesting that loss of PDI activity could exacerbate the pathology of neurodegenerative diseases (see ARF related news story).
And finally, a fascinating paper in the July 7 issue of Science shows that there is more to the UPR than upregulation of chaperones. The two major effector arms of the UPR both emanate from the ER transmembrane protein IRE-1. Sensing a build-up of unfolded proteins on its luminal side, IRE-1 activates cytosolic kinase and ribonuclease activities, which upregulate chaperone production (through RNA splicing to produce transcription factors) and downregulate protein synthesis at the level of translation. The net effect is to enhance the capacity of the ER while reducing its load. Now, Julie Hollien and Jonathan S. Weissman reveal that IRE-1 activation also leads to a rapid and specific degradation of mRNAs targeted to the ER. Using its cytosolic ribonuclease activity, IRE-1 chews up the mRNAs for the nascent polypeptides it senses in the lumen of the ER. This targeted destruction gives the ER an immediate time-out from protein folding, and also allows it to accommodate the increased syntheses of chaperones that comes later in the UPR. The elegant logic of the UPR thus revealed should only increase our appreciation of and curiosity about the role of this critical homeostatic mechanism in the health and demise of neurons.—Pat McCaffrey