In neurodegenerative diseases, misfolded proteins of various types accumulate in cells, evoking a compensatory stress response that squelches protein synthesis. Unfortunately, this attempt at damage control also compromises synapses. Growing research suggests that dampening this unfolded protein response (UPR) might protect against neurodegeneration in multiple disorders. Hot on the heels of an August report that showed the benefits of suppressing the UPR in an Alzheimer’s mouse model (see ARF related news story), a paper in the October 9 Science Translational Medicine describes similar neuroprotection in an aggressive model of prion disease. Researchers led by Giovanna Mallucci at the University of Leicester, U.K., fed prion-infected mice an inhibitor of the kinase PERK, a central player in this stress response. At the stage when untreated animals became terminally ill, the treated mice showed only mild clinical signs of disease, retaining most neurons in the brain and performing as well as wild-type mice in cognitive tasks. The finding has caught the attention of the popular press (see CBS news story; BBC news story). A major downside of the treatment is dramatic weight loss, which required the researchers to terminate experiments to comply with U.K. regulations governing animal research. Hence, they made no assessment of improved survival from the prion onslaught. The side effects would also make the compound unsuitable for use in people. Nonetheless, the data show that the UPR pathway represents a promising target for drug discovery, Mallucci told Alzforum.
“These data are really very impressive,” said Gianluigi Forloni at the Mario Negri Institute in Milan, Italy. He was not involved in the work. He noted that more studies are needed to pin down any survival effects, however, as well as to investigate whether this strategy will work in other disease models. Other commentators agreed that the results are encouraging but preliminary.
PERK (PKR-like endoplasmic reticulum kinase) phosphorylates eukaryotic initiation factor 2α (eIF2α), which regulates RNA translation. Phosphorylated eIF2α shuts down most protein synthesis in the cell, including that of synaptic proteins. AD brains and Alzheimer’s mouse models have high levels of phosphorylated eIF2α, as do mice infected with prion disease. Recently, researchers at New York University showed that genetically deleting PERK in AD mice rescued memory and behavior as well as synaptic protein levels. In prion-infected mice, Mallucci and colleagues previously showed that promoting the dephosphorylation of eIF2α rescued synapses, preserved neurons, and lengthened survival time (see Moreno et al., 2012). That study used viral vectors to manipulate protein levels in small regions of the brain, however, which would not be ideal for a human therapeutic.
In the current study, Mallucci and colleagues tested an oral PERK inhibitor, GSK2606414, developed by pharmaceutical company GlaxoSmithKline for use in cancer therapy, since PERK promotes cancer cell survival (see Axten et al., 2012). The compound enters the brain easily and in sufficient quantities to block the kinase, and is highly selective for PERK over other kinases, the authors report. First author Julie Moreno fed 50 mg/kg GSK2606414 twice daily to 20 Tg37 mice starting seven weeks after they were infected with Rocky Mountain Laboratory prion strain, or around the time synaptic loss begins. Tg37 mice overexpress cellular prion protein threefold, setting the stage for a rapidly progressing, aggressive form of disease. Twelve weeks after infection with toxic prion, the Tg37 mice developed extensive spongiform brain degeneration and began having behavioral problems. The animals treated with PERK inhibitor, however, maintained normal numbers of hippocampal neurons at 12 weeks, and had no degeneration and less astrogliosis than the controls. They also retained normal synaptic proteins, distinguished new objects from familiar ones just as well as wild-type mice, and burrowed to their heart’s content, unlike animals treated with inactive placebo. About a quarter of the GSK2606414-treated mice showed signs of early prion disease, such as rigid tails and occasional hunched posture, but none showed diagnostic features such as dragging limbs or an inability to right themselves. By contrast, all 17 of the control mice exhibited clear diagnostic signs.
The authors wondered if the treatment could preserve function in mice that were already succumbing to prion disease. They fed the compound to another group of nine Tg37 mice starting nine weeks after infection with toxic prion, when neurodegeneration and memory problems had already begun and mild symptoms were obvious. Again, the animals treated with GSK2606414 had normal hippocampal neuron counts and only minimal neurodegeneration at 12 weeks, contrasting with the severe loss seen in the mice that received placebo. Although object recognition in GSK2606414-treated mice remained poor, they recovered burrowing behavior, and developed none of the more severe symptoms present in the controls.
The treatment had systemic side effects, however. All animals given the PERK inhibitor developed mildly elevated blood glucose and lost about 20 percent of their body weight. These effects are probably due to the loss of PERK activity in the pancreas, Mallucci said, noting that PERK is a key pancreatic enzyme. PERK deficiency in humans causes diabetes as well as skeletal, liver, and kidney problems (see Gao et al., 2012). Because of the systemic side effects, this particular compound will not advance to clinical trials, Mallucci said. Rather, she sees this study as proof-of-principle that restoring protein synthesis can prevent neurodegeneration in prion disease. Other points in the pathway, such as eIF2α or its phosphatase, GADD34, might turn out to be better targets, she suggested.
Intriguingly, the beneficial effects were seen even though the treatment did nothing to restrain accumulation of misfolded prion protein. This fits with other data showing that high levels of misfolded prion protein do not necessarily result in clinical disease (see ARF related news story). “We’re targeting the downstream effects of prion on the UPR, rather than the prion protein itself,” Mallucci said. “This is important because the UPR pathway is activated by many different unfolded proteins, including those in AD and PD brains.” Next, Mallucci plans to test the PERK inhibitor in models of other neurodegenerative diseases. However, many of these models have little neuron death, making it hard to see protective effects. Forloni suggested that it would be interesting to try the inhibitor in an ALS model, since in these mice many neurons die in a relatively short time.
Commentators pointed out that it is difficult to compare the data from this study with other experimental prion therapies, because most such studies measure effects in terms of increased survival time. For example, some compounds that inhibit the accumulation of misfolded prion protein have been shown to delay disease onset by one or two months, said Markus Glatzel at University Medical Center Hamburg-Eppendorf, Germany (see, e.g., Forloni et al., 2002; Eiden et al., 2012). In future work, GSK2606414 could be delivered directly to the brain to avoid systemic side effects and obtain survival data, commentators suggested.
Other researchers cautioned that silencing the UPR over long periods might not be safe. “Translating the potential of PERK inhibitors into a viable therapeutic strategy for treating neurodegenerative diseases in humans will be challenging because PERK mediates an important adaptive response to stress,” wrote Wiep Scheper and Jeroen Hoozemans at VU University Medical Center, Amsterdam, Netherlands, in an accompanying editorial. Ralph Nixon at the Nathan Kline Institute in Orangeburg, New York, pointed out that the UPR induces autophagy, the process by which cells rid themselves of unwanted protein. Suppressing autophagy can increase Aβ levels and neurotoxicity (see Nilsson et al., 2013). Potential treatments would have to strike a balance between pumping up protein synthesis and maintaining autophagy, Nixon suggested. One solution might be to combine a PERK inhibitor with treatments that enhance autophagy or prevent the accumulation of toxic proteins specific to different diseases, researchers said. Combination therapy might also allow for milder PERK inhibition that avoids systemic side effects, Glatzel speculated.
Moreno JA, Halliday M, Molloy C, Radford H, Verity N, Axten JM, Ortori CA, Willis AE, Fischer PM, Barrett DA, Mallucci GR. Oral Treatment Targeting the Unfolded Protein Response Prevents Neurodegeneration and Clinical Disease in Prion-Infected Mice. Sci Transl Med. 2013 Oct 9;5(206):206ra138. Abstract
Scheper W, Hoozemans JJ. A New PERKspective on Neurodegeneration. Sci Transl Med. 2013 Oct 9;5(206):206fs37. Abstract
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