18 January 2011. What does the axonal protein tau have to do with synapses? Quite a bit, some scientists say. As researchers continue to dig for the roots of Alzheimer’s disease, they are turning up more evidence that tau can have powerful effects on synaptic function, learning, and memory. In the December 22 Neuron, researchers led by Dezhi Liao and Karen Hsiao Ashe at the University of Minnesota in Minneapolis show that mutant tau accumulates in dendritic spines of mouse hippocampus early in disease. This misdirected tau leads to the loss of glutamate receptors and dampens synaptic signaling. Furthermore, it is the phosphorylation of tau at specific sites that causes the protein to stray, the authors find. Meanwhile, researchers led by Erik Roberson at the University of Alabama at Birmingham and Lennart Mucke at the Gladstone Institute of Neurological Disease focused on the interactions between wild-type tau, Aβ, and the kinase Fyn. In the January 12 Journal of Neuroscience, they show that reducing tau levels in AD mice restores normal synaptic activity and network firing, adding to the evidence that tau mediates many of the toxic effects of Aβ. Both papers heighten the growing luster of tau as a therapeutic target.
Tau’s role in synapses and in AD has increasingly drawn the spotlight. Previous work by Mucke and Roberson showed that tau reduction could protect AD mice from cognitive impairment (see ARF related news story on Roberson et al., 2007), suggesting that tau acts downstream of Aβ. Intriguingly, Mucke and colleagues found that Fyn also helped Aβ poison synapses (see Chin et al., 2004; Chin et al., 2005). Last year, work by Lars Ittner and Jürgen Götz at the University of Sydney, Australia, tied the two proteins together. In mice, they showed that endogenous tau targets Fyn to the N-methyl-D-aspartic acid (NMDA) receptor, allowing tau to mediate Aβ-induced excitotoxicity at the synapse (see ARF related news story on Ittner et al., 2010). Their work implied a physiologic role for tau in dendrites. Other studies have looked at the pathologic effects of too much dendritic tau. A recent study led by Eckhard and Eva-Maria Mandelkow at the Max-Planck-Unit for Structural Molecular Biology in Hamburg, Germany, demonstrated that in cultured hippocampal neurons, Aβ treatment caused tau to wander into dendrites, leading to the loss of synaptic spines, microtubules, and mitochondria (see ARF related news story on Zempel et al., 2010). This followed earlier work by the Mandelkows in which they overexpressed tau in cultured neurons and saw the axonal protein build up in dendrites and synapses disappear. In that case, the Mandelkows traced the cause to excess tau clogging up microtubule highways (see ARF related news story on Thies et al., 2007).
Liao and Ashe were interested in looking at some of the earliest changes caused by dendritic tau. First authors Brian Hoover and Miranda Reed used the rTgP301L mouse, which produces large quantities of human tau containing the P301L mutation. The authors demonstrated that these mice display spatial memory deficits and faulty long-term potentiation (LTP) in the hippocampus at ages as young as 4.5 months, prior to any loss of synapses. In order to make sure that these effects were due to the tau mutation and not simply the high levels of tau, Hoover and Reed also created a new mouse strain (rTgWT) that expresses wild-type human tau at comparable levels to rTgP301L. These animals showed mild synaptic and memory defects, but unlike the rTgP301L cousins, no neurodegeneration or worsening of memory with age, indicating that high levels of tau by themselves do not cause disease.
Hoover and colleagues wanted to test the idea that mutant tau mislocalizes into synapses. Using brain isolates from 4.5-month-old mice, they found high levels of tau in the post-synaptic preparations of P301L mice compared to both wild-type and rTgWT mice. To directly visualize tau movements, the authors switched to an in vitro setting, transfecting primary rat hippocampal neurons with DNA for green fluorescent protein/human tau chimeras. Mutant P301L tau populated the majority of dendritic spines, while wild-type tau only rarely popped up in spines, confirming that only mutant tau invades synapses in force.
To examine if this excess tau has synaptic effects, Hoover and colleagues measured post-synaptic currents in the transfected rat hippocampal cultures as well as in cultured hippocampal neurons from the transgenic mice. In both types of cultures, the presence of P301L tau in neurons correlated with dampened miniature excitatory post-synaptic currents (mEPSCs). Wild-type human tau had no such effect. To investigate the mechanism behind this decline, the authors labeled cultured neurons with fluorescently tagged antibodies specific for several glutamate receptor subunits. The labels revealed that neurons containing mutant tau had far fewer NMDA and AMPA receptors at post-synapses than did wild-type or tau-overexpressing neurons. The authors note that this drop in receptors could explain the lower mEPSCs (see Malinow and Malenka, 2002). A loss of glutamate receptors is also known to contribute to loss of spines (see McKinney et al., 1999; Richards et al., 2005; McKinney et al., 2010), suggesting that these early effects may precede synapse loss.
Finally, Hoover and colleagues demonstrated that tau’s ability to invade synapses depends on phosphorylation. The authors focused on 14 phosphorylation sites in a proline-rich region of tau, which are normally modified by proline-directed kinases. In Drosophila, these sites have been shown to modulate the neurotoxicity of tau and to govern the binding of tau to actin, a cytoskeletal protein that also occurs in spines (see Fulga et al., 2007; Steinhilb et al., 2007; Steinhilb et al., 2007). To prevent phosphorylation of these 14 sites, Hoover and colleagues mutated the serine and threonine residues to alanine or proline; to mimic phosphorylation, they changed the residues to glutamate. When cultured rat neurons were transfected with these constructs, glutamate-modified tau invaded synapses more heavily than did wild-type tau, while the alanine-proline modified tau shunned dendritic spines.
One limitation of these experiments for AD is that they say nothing about the interaction between tau and Aβ. In future work, Ashe said, they plan to use a combined APP/tau model such as APPswe/Tauvlw to examine the relationship between these proteins. Aβ has also been shown to have post-synaptic effects, blocking synaptic plasticity and causing synapse loss (e.g., see ARF related news story on Koffie et al., 2009 and ARF related news story on Lacor et al., 2007).
“I think it’s tantalizing that the site at which amyloid-β and tau cause neuronal dysfunction appears to be the dendritic spine, at least in the earliest stages,” Ashe said. Ashe and Liao will also investigate the mechanism behind the loss of AMPA and NMDA receptors. The presence of tau might be causing increased receptor internalization, or it might be blocking the trafficking of receptors to spines, Liao speculated.
Liao believes the tau phosphorylation and mislocalization mechanism they have uncovered could show great promise for the prevention or early treatment of AD. “For a long time, people have tried to treat the late stage of AD. We believe the best strategy is to treat it early,” Liao said. The AD field as a whole is moving in this direction, based in part on poor clinical trial outcomes (e.g., see ARF related news story and ARF related news story). Liao also points out that synaptic damage is now believed to be the key cellular mechanism underlying memory loss (e.g., see ARF related news story), and that by translocating to the synapse, tau might be involved. “If we can block this unwanted guest, we can potentially find a cure for neurodegenerative disease,” he said.
Michel Goedert of the MRC Laboratory of Molecular Biology in Cambridge, U.K., agrees this approach might have promise. “[The paper] confirms that tau hyperphosphorylation could be a good therapeutic target.” However, when relating these findings to the human condition, Goedert said, it will be important to know if the tau that enters spines is soluble or already filamentous. “From the human cases, there is evidence to suggest that tau aggregates early on, and it aggregates probably in the axon, and it could be that it aggregates before mislocalization.” (See, e.g., Braak and Del Tredici, 2010.) Another unanswered question, Goedert points out, is whether the mistargeting of mutant tau depends on the very high expression levels in these mice.
Eva-Maria Mandelkow notes that the phosphorylation sites examined by Liao and colleagues are not the sites that control attachment of tau to microtubules. It would be informative to also look at the effect of phosphorylation at these sites, she said, since detachment from filaments is a necessary precursor to tau aggregation and mislocalization.
Several commentators wonder how these experiments might relate to the behavior of endogenous, wild-type tau in AD. In that regard, Fred Van Leuven of Katholieke Universiteit Leuven, Belgium, finds it interesting that Liao and colleagues show that transgenic mice overexpressing wild-type human tau do show some cognitive deficits. “This is a major finding, important for the many primary tauopathies caused by wild-type tau, and not least for AD, the most frequent secondary tauopathy,” Van Leuven wrote in an e-mail to ARF. Van Leuven also points out that “we still do not know whether tau is a physiological constituent in spines and post-synaptic compartments or whether it wanders off there only in pathological conditions.” The answer to that question would also affect treatment approaches.
In the second paper, Roberson and Mucke examined the behavior of wild-type tau in relation to Fyn and Aβ. They crossed mice having a mild AD phenotype (hAPPJ9) with animals that overexpressed mouse Fyn to create a strain that demonstrates numerous defects characteristic of AD, such as memory problems, frequent seizures, hippocampal remodeling, and early mortality. When first author Roberson crossed these mice with tau knockout mice, the synaptic and network behavior returned to normal, adding to the evidence that the negative effects of Fyn and Aβ depend on tau. Likewise, the authors showed that tau reduction also normalized synaptic transmission, NMDA receptor function, long-term potentiation, and network excitability in hAPPJ20 mice. Significantly, Roberson and colleagues showed that tau reduction did not lengthen survival in a mouse model of amyotrophic lateral sclerosis, indicating that the protective effect of ablating tau is relatively specific for AD pathology.
“This paper brings a few ideas together,” Roberson said. It shows that tau reduction not only protects against seizures, but also normalizes many other aspects of electrophysiology in AD mice. It ties tau to network dysfunction, Roberson said, suggesting that tau plays a role in regulating neuronal activity and synchrony. In AD mice, “the inhibitory interneurons do not seem to be firing enough. This produces an imbalance between excitation and inhibition, which may drive a lot of the abnormal synchronization and abnormal firing rates of the neurons.” Dialing down tau prevents this imbalance.
In future work, Roberson said, he would like to use an inducible mouse model to control the timing and location of tau reduction. He hopes this approach will allow him to dissect the mechanisms behind tau’s contribution to pathology, and figure out what therapeutic approaches might work best. “Our paper provides further evidence that reducing tau or otherwise tapping into the effects of tau reduction, perhaps by targeting the tau-Fyn interaction, is potentially a powerful therapeutic approach to the disease,” Roberson suggested.—Madolyn Bowman Rogers.
Hoover BR, Reed MN, Su J, Penrod RD, Kotilinek LA, Grant MK, Pitstick R, Carlson GA, Lanier LM, Yuan LL, Ashe KH, Liao D. Tau mislocalization to dendritic spines mediates synaptic dysfunction independently of neurodegeneration. Neuron. 2010 Dec 22;68(6):1067-81. Abstract
Roberson ED, Halabisky B, Yoo JW, Yao J, Chin J, Yan F, Wu T, Hamto P, Devidze N, Yu GQ, Palop JJ, Noebels JL, Mucke L. Amyloid-beta/Fyn-induced synaptic, network, and cognitive impairments depend on tau levels in multiple mouse models of Alzheimer’s disease. J Neurosci. 2011 Jan 12;31(2):700-711. Abstract