In neurodegenerative diseases, normal cellular waste disposal systems fail to clear misfolded proteins. One of the systems that goes awry is the proteasome, a meat-grinder-like organelle that chews up unwanted proteins marked with ubiquitin tags. In the December 21 Nature Medicine, researchers led by Karen Duff at Columbia University, New York, in collaboration with Alfred Goldberg at Harvard Medical School, make the case that hyperphosphorylated, aggregated tau directly harms proteasome function, perhaps explaining the failure of this organelle in tauopathies. In tauopathy model mice, stimulating proteasome function by activating protein kinase A cleared accumulated tau and improved learning and memory. Several drugs that activate PKA are already approved by the Food and Drug Administration.
“This is an elegant, meticulous study, and the data look compelling,” said Gerold Schmitt-Ulms at the University of Toronto. Researchers have speculated that proteasome dysfunction might play a key role in tauopathies, but they have struggled to demonstrate this in mouse models, Schmitt-Ulms noted. “The authors show something that many people have suspected, but had trouble documenting. It’s not so straightforward to activate the proteasome in vivo,” he added.
Proteasome function slows down in areas of the brains burdened with amyloid pathology (see Keller et al., 2000; Apr 2010 news). Some previous studies have correlated this dysfunction to the presence of hyperphosphorylated, misfolded tau, and found that aggregated tau inhibits proteasomes in vitro (see Keck et al., 2003; Tai et al., 2012). However, these studies stopped short of demonstrating a direct effect in vivo.
To do this, first author Natura Myeku used rTg4510 mice, which express mutant (P301L) human tau and develop tangles starting at about four months of age. She found that proteasome activity in cortical brain extracts from these mice weakened with age, in parallel with worsening tau pathology. The authors crossed rTg4510 mice with a reporter line that expresses a green fluorescent protein labeled for proteasomal destruction (see Lindsten et al., 2003). In the reporter mouse, neurons degraded the GFP quickly and barely fluoresced, but in the P301L tau crosses, green dots accumulated around tau deposits, increasing with age (see image). Moreover, proteasomes from rTg4510 mice co-immunoprecipitated with tau, indicating a physical association. The mutant protein might be obstructing and poisoning proteasomes, the author suggested.
Could normal proteasome function be restored? Elevated levels of cyclic AMP activate protein kinase A, which in turn stimulates proteasome activity by phosphorylating one of its subunits, according to some reports (see Zhang et al., 2007; Metcalfe et al., 2012; Myeku et al., 2012). To test if cAMP could rescue tau toxicity, the authors intraperitoneally injected rolipram, a phosphodiesterase 4 inhibitor that boosts cAMP levels, into three- to four-month-old rTg4510 mice for three weeks. Levels of insoluble and phosphorylated tau dropped by about a quarter, and the animals learned the location of a hidden platform in the Morris water maze as well as wild-type animals, while untreated transgenics performed poorly. Purified proteasomes from treated mice chewed up proteins better than those from untreated mice, and maintained good proteolysis even when exposed to aggregated tau in vitro, suggesting they were protected from its toxic effects.
Although rolipram treatment helped young mice in the earliest stages of tauopathy, for eight- to10-month-old mice the same regimen provided no benefit. These older mice accumulate much more aggregated tau, and the treatment might have to be longer or at a higher dose to have an effect, Myeku suggested. Alternatively, proteasomes from these late-stage mice might resist phosphorylation, she said. In ongoing work, the authors are testing additional pharmacological approaches to raise cAMP levels, some of which might be more potent. They also want to find safer drugs. Rolipram is no longer used in people because of side effects such as nausea, but other, newer drugs in its class, such as apremilast and roflumilast, have better safety profiles, Myeku said. She believes some of these might have therapeutic potential for treating Alzheimer’s disease or other tauopathies.
In addition, some evidence hints that stimulation of proteasomes could provide broad protection against other neurodegenerative diseases. Goldberg recently reported that boosting cAMP in cell lines revved up proteasomes and cleared mutant FUS, SOD1, and TDP-43 in addition to tau (see Lokireddy et al., 2015). “We’re very interested in testing this mechanism of action in mouse models of other neurodegenerative diseases,” Myeku said.
Meanwhile, the details of how tau harms proteasomes remain unclear. The authors speculate that mutant tau may obstruct the access of other proteins to the proteasome, clogging up the machinery. This model implies that mutant tau binds more strongly to proteasomes than does wild-type. However, Schmitt-Ulms recently found the opposite: In a direct comparison, P301L tau bound more weakly to proteasomes than wild-type tau did, leading him to a different interpretation. “We prefer a model whereby the interaction between mutant P301L tau and the proteasome is partially impaired relative to wild-type tau, perhaps contributing to poor proteasome processivity for this substrate. Over time, these altered properties of tau may lead to its aggregation,” he wrote to Alzforum. This aggregated tau may then bind the proteasome and poison it, as Myeku et al. suggest, he added (see full comment below).—Madolyn Bowman Rogers
Research Models Citations
- Keller JN, Hanni KB, Markesbery WR. Impaired proteasome function in Alzheimer's disease. J Neurochem. 2000 Jul;75(1):436-9. PubMed.
- Keck S, Nitsch R, Grune T, Ullrich O. Proteasome inhibition by paired helical filament-tau in brains of patients with Alzheimer's disease. J Neurochem. 2003 Apr;85(1):115-22. PubMed.
- Tai HC, Serrano-Pozo A, Hashimoto T, Frosch MP, Spires-Jones TL, Hyman BT. The synaptic accumulation of hyperphosphorylated tau oligomers in Alzheimer disease is associated with dysfunction of the ubiquitin-proteasome system. Am J Pathol. 2012 Oct;181(4):1426-35. PubMed.
- Lindsten K, Menéndez-Benito V, Masucci MG, Dantuma NP. A transgenic mouse model of the ubiquitin/proteasome system. Nat Biotechnol. 2003 Aug;21(8):897-902. Epub 2003 Jul 20 PubMed.
- Zhang F, Hu Y, Huang P, Toleman CA, Paterson AJ, Kudlow JE. Proteasome function is regulated by cyclic AMP-dependent protein kinase through phosphorylation of Rpt6. J Biol Chem. 2007 Aug 3;282(31):22460-71. Epub 2007 Jun 12 PubMed.
- Metcalfe MJ, Huang Q, Figueiredo-Pereira ME. Coordination between proteasome impairment and caspase activation leading to TAU pathology: neuroprotection by cAMP. Cell Death Dis. 2012 Jun 21;3:e326. PubMed.
- Myeku N, Wang H, Figueiredo-Pereira ME. cAMP stimulates the ubiquitin/proteasome pathway in rat spinal cord neurons. Neurosci Lett. 2012 Oct 11;527(2):126-31. Epub 2012 Sep 5 PubMed.
- Lokireddy S, Kukushkin NV, Goldberg AL. cAMP-induced phosphorylation of 26S proteasomes on Rpn6/PSMD11 enhances their activity and the degradation of misfolded proteins. Proc Natl Acad Sci U S A. 2015 Dec 29;112(52):E7176-85. Epub 2015 Dec 15 PubMed.
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- Myeku N, Clelland CL, Emrani S, Kukushkin NV, Yu WH, Goldberg AL, Duff KE. Tau-driven 26S proteasome impairment and cognitive dysfunction can be prevented early in disease by activating cAMP-PKA signaling. Nat Med. 2016 Jan;22(1):46-53. Epub 2015 Dec 21 PubMed.