In the Alzheimer’s brain, tau dissociates from microtubules, becomes hyperphosphorylated, and aggregates into neurofibrillary tangles. What kicks off this process? In the October 6 Neuron, researchers led by Huda Zoghbi and Juan Botas at the Jan and Dan Duncan Neurological Research Institute at Texas Children’s Hospital, Houston, indict a new suspect. The kinase Nuak1, which structurally resembles AMP-activated protein kinase (AMPK), phosphorylates serine 356 in tau’s microtubule-binding domain, and this modification prevents tau degradation, causing levels to build up in the cytoplasm. That in turn triggers hyperphosphorylation, the authors report. Cutting Nuak1 levels in half in a tauopathy mouse model ameliorated pathology, restored normal synaptic plasticity, and improved memory. The findings point toward a new strategy for tackling tau accumulation, Zoghbi suggested.
Others agreed. “Therapeutic targeting of AMPK-related kinases … may offer a new way to combat AD and related tauopathies, for which there are currently no effective treatment options,” Bingwei Lu at Stanford University, Palo Alto, California, wrote to Alzforum. Franck Polleux at Columbia University, New York City, called the paper exciting. “The authors present strong evidence that Nuak1 is an important tau kinase, and may be a promising therapeutic target,” he said.
Zoghbi’s earlier work on spinocerebellar ataxias convinced her that protein levels matter in neurodegeneration, she told Alzforum. Previously, she had reported that lowering ataxin1 protein in a mouse model of the disease protected neurons and improved motor skills (see Park et al., 2013). Zoghbi wondered if the same approach might help in tauopathies such as Alzheimer’s disease, since previous work associated less tau with better memory in AD mice (see May 2007 news).
To find factors that influence the amount of tau, Zoghbi’s group focused on kinases. They screened all known human kinases by knocking them out one by one with RNAi in a human cell line, and measuring the resulting tau levels. Meanwhile, Botas’ group ran a similar screen in fruit flies that overexpress human tau, looking for kinases that lessened degeneration in the eye. The authors identified 16 kinases that affected both systems. From these, they selected Nuak1 for follow-up based on its particularly robust effects. Nuak1 was not previously known to interact with tau. Past screens for tau modifiers have mostly focused on hyperphosphorylation or aggregation of the protein, rather than abundance, which may be why this kinase did not show up in those studies, the authors noted.
First author Cristian Lasagna-Reeves turned to in vitro assays to validate Nuak’s interaction with tau, finding that it selectively acted on only Ser356. This site and its neighbor Ser262 have been dubbed “priming sites,” because their phosphorylation leads tau to detach from microtubules and subsequently become hyperphosphorylated. In keeping with this result, Lasagna-Reeves and colleagues found that overexpression of Nuak1 in neuroblastoma cells pumped up Ser356 phosphorylation as well as levels of tau and hyperphosphorylated tau, while depleting Nuak1 did the opposite. The increase in overall phosphorylation supports the idea that phosphorylation at Ser356 kicks off a cascade, the authors note.
Why did tau levels soar? The authors saw no difference in tau mRNA levels, indicating no change in gene expression. Instead, tau phosphorylated at Ser356 hung around longer. Using the cell assays, the authors found that phosphorylation at this site prevented the ubiquitinase CHIP from binding and tagging the protein for degradation, as it normally does (see Dickey et al., 2007; Dickey et al., 2008). Meanwhile, cells that expressed a mutant version of tau lacking the Ser356 site were unaffected by overexpression of Nuak1.
To probe Nuak1 in a disease model, the authors crossed Nuak1 heterozygous knockouts with P301S mice, which express human mutant tau at five times endogenous levels. These animals develop neurofibrillary tangles by six months of age. However, the Nuak1-deficient mice sported less pathological tau than their P301S littermates at 7.5 months, having about one-third as many neurons that contained paired helical filaments (see image above). In addition, their total tau levels were down about 20 percent, and they had about half as much tau phosphorylated at Ser356.
The changes in tau came with altered behavior, too. At 7.5 months old, Nuak1-deficient mice remembered where the hidden platform was in the Morris water maze better than their P301S littermates did, although less well than controls. Likewise, they froze more than PS01Ss in an environment where they had previously received a shock, though again not to control levels. Both these tasks depend on the hippocampus. The authors tested hippocampal function directly by implanting electrodes and measuring synaptic plasticity in live, active mice. While P301S animals were impaired, their Nuak1-deficient littermates were indistinguishable from controls.
Does Nuak1 play a role in human disease? In postmortem brain tissue, the authors detected twice as much Nuak1 protein in AD brains as in healthy controls. They also measured double the normal Nuak1 in brains from people who had had progressive supranuclear palsy, a different tauopathy, suggesting elevated Nuak1 may be common to tauopathies. S356 phosphorylation by Nuak1 may be broadly activated in different neurodegenerative diseases, they posit. In addition, Nuak1 localized to neurofibrillary tangles in both conditions (see image above).
Polleux found the human data intriguing, noting that the findings mirror those for AMPK. He had previously reported that AMPK phosphorylates tau at Ser262, and that this kinase associates with tau tangles in AD brains. AD brains contain an abundance of the activated form of AMPK, Polleux and colleagues reported. In mice, they uncovered evidence that Aβ oligomers switch on AMPK (see Apr 2013 news). It would be interesting to know if Aβ also overactivates Nuak1, or other members of the same kinase family, Polleux said. Lasagna-Reeves plans to study Nuak1 and Aβ interactions by crossing the heterozygous Nuak1 knockouts with an APP mouse model.
Meanwhile, Nuak1 might make an attractive therapeutic target, Zoghbi agreed. There are no drugs that inhibit Nuak1, and researchers would have to be careful not to suppress it too much to spare potentially crucial functions. For example, Nuak1 controls terminal axon branching during development (see Courchet et al., 2013). More broadly, Zoghbi believes the general strategy of lowering tau may be useful. She is following up on other kinases uncovered in this screen, and plans to screen a much broader group of proteins for their effects on tau as well.—Madolyn Bowman Rogers
- APP Mice: Losing Tau Solves Their Memory Problems
- Tracing a Path from Aβ to Tau Leads Scientists to Lost Synapses
Research Models Citations
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- Protecting Proteasomes from Toxic Tau Keeps Mice Sharp
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- Therapeutic Approaches Target Deubiquitinase, Protein Turnover
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