When neurons stress out, they put the kibosh on protein production until conditions improve. If the stress persists, the shutdown can be disastrous for synaptic function. Now, a study led by Sergio Ferreira at the Federal University of Rio de Janeiro and Eric Klann at New York University reports that a small molecule inhibitor of the integrated stress response, called ISRIB, counteracted neuronal stress in the face of Aβ oligomers or rampant amyloidosis in mice. The treatment prevented synaptic deficits and memory loss, and even restored memory in plaque-ridden mice.
- The small molecule inhibitor ISRIB counteracted stress response in mice.
- ISRIB prevented synaptic and memory loss in Aβ toxicity model.
- In amyloid-ridden mice, ISRIB restored synaptic function and memory.
The study, published February 2 in Science Signaling, adds to a growing body of evidence implicating a stress-induced slump in protein production in neurodegeneration, and further increases confidence in targeting this pathway for treatment of Alzheimer’s and related disorders, noted Giovanna Mallucci of Cambridge University in Cambridge, England, U.K.
The integrated stress response (ISR) exists for a good reason—to allocate precious cell resources to the most crucial tasks. One way the cell does this is by shutting down the bulk of new protein synthesis, allowing continued production of only essential proteins. Processes that rely on constant turnover of new proteins—including synaptic signaling and plasticity—take a hit (Trinh and Klann, 2013).
The shutdown begins when stress-induced kinases, including PERK, PKR, and GCN2, phosphorylate the alpha subunit of elongation initiation factor-2 (eIF2α). In its unphosphorylated form, eIF2α functions as part of a heterotrimeric complex along with eIEF2β —a guanine nucleotide exchange factor—and eIF2γ, to initiate translation. However, once phosphorylated, eIF2α blocks GTP from binding to eIF2β, grinding protein synthesis to a halt. Phosphorylated eIF2α has been detected in the brains of people with neurodegenerative diseases, including AD, PD, and ALS. Previously, Ferreira, Klann and colleagues reported that suppression of eIF2α kinases alleviated deficits in synaptic plasticity and memory in mouse models of Aβ toxicity (Aug 2013 news; Lourenco et al., 2013).
Because kinases are notoriously difficult to target, and there are four of them that conspire to phosphorylate eIEF2α, the authors decided to disable the ISR downstream. They used ISRIB, a small molecule that binds eIF2β, preventing its deactivation by phosphorylated eIF2α (Sidrauski et al., 2013; Zyryanova et al., 2018). Previous studies reported that the inhibitor stemmed neuronal damage in mouse models of cellular stress, prion disease, Down’s syndrome, and traumatic brain injury (Sidrauski et al., 2015; Halliday et al., 2015; Zhu et al., 2019; Chou et al., 2017).
Before testing the inhibitor in mouse models, first author Mauricio Oliveira and colleagues confirmed previous reports that the ISR was overactivated in the AD brain. Using western blot to probe cortical extracts from brain samples of eight people with AD, the researchers detected nearly double the amount of phosphorylated eIF2α as in samples from eight controls. In addition, AD brain samples had only half as much eIF2β as controls. The findings indicated that indeed, the ISR was elevated in the AD brain.
Could disabling the ISR prevent damage caused by Aβ oligomers? Previous studies had demonstrated that Aβ oligomers not only induce the stress response, but also hinder synaptic function and cause memory loss in mice. Here, the researchers reported that after injecting a single dose of 10 pmol Aβ oligomers into the cerebral ventricles of wild-type mice, the density of dendritic spines in the hippocampus crept downward over 12 days, and the mice began to forget the environment in which they had previously received a foot shock. Daily intraperitoneal injections of ISRIB—started on the same day as the oligomer injection—prevented both the spine loss and memory deficits. The researchers also found that the injected Aβ oligomers induced the ISR—as gauged by eIF2α phosphorylation and reduced protein synthesis, among other indicators—and that treatment with ISRIB prevented ISR activation downstream of eIF2α phosphorylation, and kept protein production up and running.
Might ISRIB treatment pass the higher bar of reversing damage in older mice burdened with Aβ plaques? To find out, the researchers treated 10- to 13-month-old APP/PS1 and wild-type mice for two weeks with daily doses. Compared to wild-type animals, APP/PS1 mice had about 20 percent fewer dendritic spines in the hippocampus. ISRIB restored spine density to nearly wild-type levels, but had no effect on spine density in wild-type mice. The researchers also found that treating hippocampal slices from APP/PS1 mice with ISRIB corrected defects in long-term potentiation, a measure of synaptic plasticity. These synaptic benefits translated into a cognitive boost. ISRIB-treated APP/PS1 mice performed at wild-type levels on spatial learning and contextual memory tasks.
Although squelching the stress response restored synaptic function and memory, it did not rid the mice of Aβ plaques. ISRIB-treated APP/PS1 mice had just as many as did untreated mice, although they were smaller and denser. The researchers speculated that changes in microglial function could underlie this last finding, though there were no overt changes in microglial number or Iba1expression with treatment. In all, the findings suggest that ISRIB can restore synaptic function even in the continued presence of substantial amyloid deposition.
The findings also hint that low, repeated doses of ISRIB might boost protein synthesis without causing side effects. This would be welcome news, because in previous studies in mouse models of amyloidosis, one dose was ineffective and daily doses of 5 mg/kg—20 times higher than what Oliveira and colleagues used—were toxic (Briggs et al., 2017; Johnson and Kang, 2016). PERK inhibitors—which block the ISR upstream of eIF2α—are also toxic (Halliday et al., 2015).
“The paper points to dysregulation of protein synthesis as a cause of synapse loss and memory impairment in AD,” commented Peter Giese of King’s College London. “This work also suggests that synapse numbers and learning abilities can be restored by compounds like ISRIB in the early stages of the disease.” He added that the mouse models used in the study do not develop tau pathology, so more work is needed to establish the validity of the approach in people with AD.
“Converging evidence suggests that the ISR may be a central molecular switch for memory consolidation, applicable to a wide range of neurodegenerative disorders and diseases, brain injury, and aging,” commented Susanna Rosi and Elma Frias of the University of California, San Francisco. “These findings strengthen the idea that targeting the ISR may represent an effective therapeutic strategy to ameliorate AD-associated memory deficits.”
They noted that Calico Life Sciences, which recently licensed ISRIB, will begin a Phase 1 trial, partnered with Abbvie, of its eIF2β activator, ABBV-CLS-8262, in people with ALS this year (press release).—Jessica Shugart
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