Neurofibrillary tangles, one of the toxic protein aggregates found in Alzheimer's disease (AD), comprise accumulations of the microtubule protein tau. How might those proteins be kept apart? Tiny sugars might do just that, suggests a paper in the February 26 Nature Chemical Biology online edition. For the first time, David Vocadlo, from the Simon Fraser University, Burnaby, British Columbia, Canada, and colleagues reported that chronically promoting a form of glycosylation in tau transgenic mice reduces neurofibrillary tangles and neurodegeneration. Contrary to previous results, however, the sugar seems to work without interfering with tau hyperphosphorylation, which promotes tau aggregation. The findings suggest that sugar moieties on tau—and perhaps on other proteins—prevent them from clumping together. "It suggests that one potential contributing factor to the aggregation of tau is the level of sugar modification inside cells," Vocadlo told ARF. "Perhaps this is a mechanism that could be exploited to prevent the pathological formation of toxic tau species." Another potential approach might be to inhibit modification of tau by acetylation. In the March 3 issue of the journal Brain, researchers from John Trojanowski’s lab at the University of Pennsylvania, Philadelphia, report that a specific, acetylated tau accompanies hyperphosphorylated forms of the protein in intracellular inclusions in several tauopathies, including AD. These authors suggest that this acetylated variant helps drive tau polymerization, injecting yet another form of post-translational modification into the process going from native to toxic tau.

Addition of N-acetylglucosamine (O-GlcNAc) to protein hydroxyl groups is a post-translational modification made to thousands of proteins in mammals. In many cases, addition of the sugar occurs at phosphorylation sites, and the two processes are thought to compete with one another (for a review, see Hart et al., 2011). In the brains of people with AD, hyperphosphorylated tau is mirrored by a decline in O-GlcNAc-modified tau (see Liu et al., 2009). One reason for the reduced glycosylation, which depends on adequate nutrient levels, could be alterations in glucose metabolism (see Schubert, 2005). Vocadlo and colleagues decided to test whether increasing O-GlcNAc modification of proteins would protect neurons and guard against neurofibrillary tangle (NFT) formation in a mouse model of tau neurodegeneration.

Co-first authors Scott Yuzwa and Xiaoyang Shan laced the drinking water of nine- to 12-week-old hemizygous JNPL3 mice with a compound developed in Vocadlo's lab. Thiamet-G inhibits the glycoside hydrolase (O-GlcNAcase) that removes N-acetylglucosamine from proteins. The transgenic mice carry the gene for human tau with the P301L mutation and develop neurodegeneration in the brain. Twenty mice imbibed the spiked water for 36 weeks, while 20 control mice lapped up untreated water.

JNPL3 mice show dramatic motor neuron loss in their spinal cords—ending up with about half the number that wild-type mice have—so the researchers first checked there for neuron rescue. At the end of the 36 weeks, thiamet-G-treated mice retained 1.4-fold more motor neurons in their cervical spinal cords than did untreated mice. The treated mice also maintained more of their body weight, losing less skeletal muscle than is common in this model.

What led to the protection? The AT8 antibody, which recognizes phosphorylated tau, detected 23 to 62 percent less phospho-tau immunoreactivity around neurofibrillary tangle-like structures in mouse brainstem, spinal cord, hypothalamus, and cortices. At the same time, O-GlcNAc jumped four- to fivefold in the brainstems of treated mice compared to controls. It seemed the neurons benefited from fewer NFTs, presumably through reduced phosphorylation, as previous research suggests. However, again using AT8 antibody, the researchers found no change in the global tau phosphorylation levels (including soluble forms) in homogenized JNPL3 brain tissue compared to control samples. That result led the team to hypothesize that added sugars reduced tau's clumping into tangles without blocking its phosphorylation sites. "One of O-GlcNAc's major functions might be to serve as a bumper to keep proteins from bumping into each other and aggregating," said Gerald Hart, Johns Hopkins University School of Medicine, Baltimore, Maryland, told ARF. Hart was not involved in the study.

To further test whether the added sugar reduced aggregation without hindering phosphorylation, the team did in-vitro tests on a truncated, fast-aggregating tau snippet. Glycosylating the snippet slowed that peptide's aggregation even in the absence of a phosphorylation effect. "We were surprised," Vocadlo told ARF. "It shows that the sugar modification on its own can influence the aggregation of tau, at least in vitro, in the absence of other factors, including phosphorylation." Vocadlo's team showed in a previous study that acutely treating wild-type rats with thiamet-G for a single day did reduce tau phosphorylation, while boosting the addition of O-GlcNAc to tau (see ARF related news story on Yuzwa et al., 2008). However, cells may adapt to treatment with chronic exposure, said Vocadlo, which could explain why continuous treatment of the JNPL3 mice is not accompanied by a reduction in tau phosphorylation. Since in this study, O-GlcNAc also hindered the heat-induced accumulation of an unrelated protein, the sTAB1 binding protein, the authors suggested that a general role of O-GlcNAc may be to prevent protein aggregation.

If glycosylation counteracts aggregation, what promotes it? Here, acetylation might come in, bespeaking the complexity of tau biology (see ARF related news story; also Min et al., 2010). In their new paper, the Trojanowski group extended earlier work showing that acetylation led to both a loss of one of tau’s major functions, i.e., promoting microtubule assembly, and a gain of toxic function, i.e., pathological tau aggregation. First author David Irwin and colleagues examined, postmortem, brains of people who had AD, corticobasal degeneration, or progressive supranuclear palsy, though no cognitively normal controls. They report that tau acetylated at lysine 280 appeared in similar structures as hyperphosphorylated tau in all cases. While acetylated tau appeared at all stages of AD, it was most prevalent in middle and late stages of the disease. Hence, it appears to occur mostly after tau phosphorylation, which may actually open the door for the acetylase to modify the protein, suggest the authors. They point out that most phosphorylation sites in tau flank the microtubule-binding repeat where lysine 280 sits. The UPenn authors suggest that acetylation may exacerbate loss of normal tau function and foster fibrillization, offering a new therapeutic target for AD and other tauopathies. If glycosylation modified those same phosphorylation sites, as has been proposed, it might also protect tau against subsequent acetylation. "We are interested in the potential interaction between different modifications on tau, but have not looked at the effects of acetylation on tau aggregation," Vocadlo told ARF in an e-mail.

Cheng-Xin Gong, New York State Institute for Basic Research, Staten Island, is not completely convinced of Vocadlo and colleagues’ finding that O-GlcNAc has no effect on tau phosphorylation. Since the researchers did not examine all of tau’s phosphorylation sites in their tests, other sites could have been modified, he pointed out. While further study is warranted on that point, "it is possible this drug may work through different pathways to benefit people with AD," he said, referencing a study suggesting that O-GlcNAc addition steers APP processing away from the amyloidogenic pathway (see Jacobsen and Iverfeldt, 2011). Recent research also suggests that O-GlcNAc plays a direct role in learning and memory, Hart pointed out (see Rexach et al,. 2012 and Kaleem et al., 2011). The Canadian team did not characterize cognition in these mice because they don't show the cognitive defects that other models do, Vocadlo said. As for motor function, treated mice fared no better than untreated. In the cage hang test of motor skills and coordination, untreated mice declined faster than treated mice, but the treated mice hung on for shorter times overall, possibly because they weigh more, so results are inconclusive, the authors note. In the rotarod test of strength and balance, both groups performed equally. The scientists plan to search for other inhibitors of O-GlcNAc addition, and test thiamet-G in other animal models to further probe its effects, Vocadlo said.

Proteins related to DNA transcription, protein translation, trafficking and degradation, cancer, and the cytoskeleton are all glycosylated and regulated by O-GlcNAc. The sugar's far-reaching influence prompts concerns about side effects when tilting the balance of glycosylation. For example, excess O-GlcNAc protein modification may worsen insulin resistance (see Vosseller et al., 2002). The team saw no negative effects on weight, food consumption, or motor neuron counts in wild-type mice treated with thiamet-G over 22 weeks. "This is hard to believe," given O-GlcNAc's range of effects, said Gong. He was unsure why that might be, but said that perhaps glycosylation less dramatically contorts protein conformation than phosphorylation, or that the body somehow buffers the compound's effects.

"What is important here is they have shown that, in vivo, thiamet-G can reduce tau pathology," said Khalid Iqbal, New York Institute for Basic Research in Developmental Disabilities, Staten Island. Before the compound can be considered for human clinical trials, researchers will need to look for cognitive benefits in other animal models of tau pathology, especially ones that more closely mimic the disease mechanisms of sporadic AD, he said. The team will also need to do further work to elucidate the mechanism of thiamet-G, especially whether it has effects on phosphorylation, researchers agreed.—Gwyneth Dickey Zakaib.

References:
Yuzwa S, Shan X, Macauley MS, Clark T, Skorobogatko Y, Vosseller K, Vocadlo DJ. Increasing O-GlcNAc slows neurodegeneration and stabilizes tau against aggregation. Nature Chem Bio 2012 February 26; Advance Online Publication. Abstract

Irwin DJ, Cohen TJ, Grossman M, Arnold SE, Xie SX, Lee VM-Y, Trojanowski JQ. Acetylated tau, a novel pathological signature in Alzheimer's disease and other tauopathies. Brain. 2012 March;135 (3): 807-818. Abstract

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References

News Citations

  1. Target Practice: A Trio of Papers to Ponder for Potential Therapies
  2. Tau Modification—Move Over Phosphate, Make Room for Acetylation

Paper Citations

  1. . Cross talk between O-GlcNAcylation and phosphorylation: roles in signaling, transcription, and chronic disease. Annu Rev Biochem. 2011 Jun 7;80:825-58. PubMed.
  2. . Reduced O-GlcNAcylation links lower brain glucose metabolism and tau pathology in Alzheimer's disease. Brain. 2009 Jul;132(Pt 7):1820-32. PubMed.
  3. . Glucose metabolism and Alzheimer's disease. Ageing Res Rev. 2005 May;4(2):240-57. PubMed.
  4. . A potent mechanism-inspired O-GlcNAcase inhibitor that blocks phosphorylation of tau in vivo. Nat Chem Biol. 2008 Aug;4(8):483-90. PubMed.
  5. . Acetylation of tau inhibits its degradation and contributes to tauopathy. Neuron. 2010 Sep 23;67(6):953-66. PubMed.
  6. . O-GlcNAcylation increases non-amyloidogenic processing of the amyloid-β precursor protein (APP). Biochem Biophys Res Commun. 2011 Jan 21;404(3):882-6. PubMed.
  7. . Dynamic O-GlcNAc modification regulates CREB-mediated gene expression and memory formation. Nat Chem Biol. 2012;8(3):253-61. PubMed.
  8. . CREB in long-term potentiation in hippocampus: role of post-translational modifications-studies In silico. J Cell Biochem. 2011 Jan;112(1):138-46. PubMed.
  9. . Elevated nucleocytoplasmic glycosylation by O-GlcNAc results in insulin resistance associated with defects in Akt activation in 3T3-L1 adipocytes. Proc Natl Acad Sci U S A. 2002 Apr 16;99(8):5313-8. PubMed.
  10. . Increasing O-GlcNAc slows neurodegeneration and stabilizes tau against aggregation. Nat Chem Biol. 2012 Apr;8(4):393-9. PubMed.
  11. . Acetylated tau, a novel pathological signature in Alzheimer's disease and other tauopathies. Brain. 2012 Mar;135(Pt 3):807-18. PubMed.

External Citations

  1. JNPL3 mice

Further Reading

Papers

  1. . O-GlcNAcylation regulates phosphorylation of tau: a mechanism involved in Alzheimer's disease. Proc Natl Acad Sci U S A. 2004 Jul 20;101(29):10804-9. PubMed.
  2. . Phosphoinositide signalling links O-GlcNAc transferase to insulin resistance. Nature. 2008 Feb 21;451(7181):964-9. PubMed.
  3. . Elevated nucleocytoplasmic glycosylation by O-GlcNAc results in insulin resistance associated with defects in Akt activation in 3T3-L1 adipocytes. Proc Natl Acad Sci U S A. 2002 Apr 16;99(8):5313-8. PubMed.
  4. . Chemical approaches to understanding O-GlcNAc glycosylation in the brain. Nat Chem Biol. 2008 Feb;4(2):97-106. PubMed.
  5. . Reduction of O-linked N-acetylglucosamine-modified assembly protein-3 in Alzheimer's disease. J Neurosci. 1998 Apr 1;18(7):2399-411. PubMed.

News

  1. Dynamic Glycosylation in Neurons May Be Linked to Memory, Pathology
  2. New Insights into Managing Misfolded Proteins
  3. Target Practice: A Trio of Papers to Ponder for Potential Therapies
  4. Tau Modification—Move Over Phosphate, Make Room for Acetylation