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FOR DECADES, the sticky amyloid-β peptide that accumulates into plaques around brain cells has garnered consider-able attention from researchers who study Alz heimer’s disease. Despite years of labor and millions of dollars invested in amyloid research, however, the neurodegeneration caused by Alz heimer’s remains essentially untreatable, although several candidate treatments aimed at amyloid-β are now in clinical trials (CE&N, April 5, page 12).

Meanwhile, scientists are increasingly turning their attention to another culprit in this grim disease: the neuronal protein known as tau. Researchers in both aca-demia and industry are studying the impact of malfunctioning tau and also searching for treatments to slow or even reverse its harmful effects on the brain.

“The ultimate result of tau dysfunction is that neurons lose their connections to other neurons, and when neurons are no longer communicating, that has profound effects on cognition—the ability to think and reason,” says Travis Dunckley, who studies neurodegenerative diseases at the Translational Genomics Research Insti-tute, in Phoenix.

In addition to Alz heimer’s, tau pathol-ogy is associated with other “tauopathies” including frontotemporal dementia and progressive supranuclear palsy. It can also appear in Parkinson’s patients. In Alz-heimer’s and most other tauopathies, mal-functioning tau accumulates into so-called paired helical filaments, which ultimately aggregate into neurofibrillary tangles.

Biochemist Marc W. Kirschner and

colleagues first isolated tau in 1975 at Princeton University. The protein helps assemble and stabilize microtubules in neurons within the central nervous system. Neurons “require microtubule assembly for the growth and integrity of axons and dendrites and for the transport of molecu-lar cargo between the cell body”—which contains the nucleus—“and distant syn-apses,” Harvard University biochemist and neuroscientist Michael S. Wolfe says.

Tau’s interaction with a microtubule is controlled by cycles of phosphorylation and dephosphoryla-tion at specific sites on the tau molecule. When dephospho-rylated by a phos-phatase, tau binds to the exterior of the microtubule, help-ing to stabilize it, in part by offsetting the microtubule’s nega-tive charge with its own positive charge, notes Michael K. Lee, a neuroscientist who codirects the Center for Neurodegenera-tive & Neuromuscu-lar Diseases at the University of Minne-sota, Twin Cities.

Tau phosphoryla-tion by some kinases can reduce binding

of tau to the microtubules, enabling the cell to regulate and remodel the microtu-bules when needed, Lee adds. In a healthy cell, the microtubule-free tau is rapidly degraded. This natural ebb and flow con-tributes to neurons’ ability to establish new connections and modify old ones, both during development and during learning in mature animals.

Under disease conditions, however, the process becomes unbalanced and tau be-comes excessively phosphorylated. Hyper-phosphorylated tau permanently dissociates from microtubules and begins to aggregate, says Donna M. Wilcock, a neuroscientist at Duke University Medical Center. Without tau’s support, the microtubules’ delivery of intracellular cargo can become disrupted.

“We don’t really know whether this is an effect of too much kinase activity or too little phosphatase activity,” she says. “Some-thing appears to throw the balance between the kinases and the phosphatases off.

“There are also certain phosphorylation sites along tau that seem to be more patho-logical, so when you get phosphorylation at those sites, that seems to be more closely associated with disease,” Wilcock says. She notes that some mutations associated with inherited forms of frontotemporal dementia affect those same sites and make

them much easier to phosphorylate.

Among the fac-tors that appear to disrupt proper tau phosphorylation are heavy metals. Heavy metals promote Alz-heimer’s, according to Ashley I. Bush, a neuroscientist at the University of Melbourne, in Aus-tralia, and Rudolph E. Tanzi, a neuro-scientist at Harvard and Massachusetts

SCIENCE & TECHNOLOGY

TAU TROUBLE Excess tau in those with Alz heimer’s harms cells by interfering with transport of cargo such as mitochondria along microtubules. In these neuroblastoma cells, overexpression of tau (right) prevents the normal distribution (left) of mitochondria (red).

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TIME FOR TAULong overshadowed by amyloid-β, TAU PROTEIN is gaining

ground as a therapeutic target in Alz heimer’s disease SOPHIE L. ROVNER , C&EN WASHINGTON

DANGEROUS LIAISON Diseased tau forms paired helical filaments (shown), which accumulate into neurofibrillary tangles.

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General Hospital, in Charlestown. Univer-sity of California, Irvine, neurobiologist Frank M. LaFerla has been investigating this connection. He recently reported that chronic copper exposure accelerates tau phosphorylation—possibly through kinase activation—and amyloid-β production in mice ( J. Neurochem. 2009, 108, 1550).

GLYCOSYLATION, which attaches sugars to tau, also appears to promote hyper-phosphorylation and has been detected in Alz heimer’s cases, Leonard Petrucelli, a molecular neuroscientist at Mayo Clinic’s Jacksonville, Fla., facility, notes in a review about tau ( Mol. Neurodegener. 2009, 4, 13).

Another factor that can increase phos-phorylation is aging-associated oxidative damage to kinases, Lee notes. Oxidation can also damage tau directly, making it less able to bind to microtubules and more prone to aggregate, he adds.

Furthermore, cells become less able to dispose of damaged tau as they age. As tau aggregates accumulate, they may block the intracellular transport of other biomol-ecules needed for the proper functioning of neurons, Petrucelli says.

In sum, there are numerous ways for tau to go bad and to disrupt its neuronal host. Complicating the situation further, tau trouble doesn’t develop in isolation. In many Alz heimer’s patients, for example, tau pathology is likely preceded and indeed ex-acerbated by amyloid-β pathology, suggests Einar M. Sigurdsson, a neuroscientist at New York University School of Medicine. In turn, tau pathology may make neurons more vulnerable to damage by amyloid-β.

Sigurdsson thinks accumulation of amyloid-β inside or outside neurons damages synapses, which start to retract. Hyperphosphorylated tau then begins to build up inside the afflicted neurons, and the synapses deteriorate further. The cells eventually die, leaving behind insoluble tau tangles that serve as a kind of tombstone for the vanished neurons, Wilcock says.

Amyloid-β and tau appear to share some characteristics. Amyloid-β plaques were for many years thought to be the most harmful form of amyloid-β. But an increas-ingly popular notion posits that amyloid plaques may represent the end result of the brain’s effort to collect and seques-ter smaller but more toxic aggregates of amyloid-β, such as oligomers. Likewise, some researchers believe that soluble ag-gregates of tau oligomers are more likely to lead to neuronal dysfunction and death

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38WWW.CEN-ONLINE.ORG MAY 3, 2010

than insoluble helical fragments and tan-gles ( Nature, DOI: 10.1038/nature08890). Harvard neurologist Bradley T. Hyman; University of Minnesota, Twin Cities, neu-rologist Karen Hsiao Ashe; and others are presently trying to identify which tau ag-gregates are especially neurotoxic.

Regardless of which form of amyloid-β and tau prove to be most harmful, “tau pa-thology appears to correlate better with the degree of dementia than amyloid-β pathol-ogy,” Sigurdsson says. Furthermore, reduc-ing tau production in a mouse model of Alz heimer’s disease protected the animals from developing cognitive impairments ( Science 2007, 316, 750).

That makes tau an attractive target for treatment. In fact, many drug companies are already pursuing it, as reported in a C&EN blog post (“CENtral Science,” The Haystack, April 8).

MEANWHILE, researchers in academia are also hunting for tau treatments.

The protein offers multiple targets for these investigators. Neurons produce six “isoforms” of the protein by varying the splicing of the precursor messenger RNA (pre-mRNA) from which tau is translated. Some mutations in the tau gene increase the production of isoforms containing four rather than three microtubule-binding regions, an alteration that is sufficient to cause dementia. With the goal of influenc-ing splicing to adjust the isoform ratio back to normal, Wolfe is investigating com-pounds that bind to tau’s pre-mRNA, in-cluding analogs of the anticancer drug mi-

toxantrone ( J. Med. Chem. 2009, 52, 6523). Another option for treating tauopathies

is to increase the activity of phosphatases, which strip phosphate groups from tau, Wilcock says. Phosphatases are normally held in check by protein inhibitors that are bound to these enzymes, she notes, so the best approach might be to somehow re-move those inhibitors.

Researchers are also trying to damp down the activity of kinases that phospho-rylate tau, Wilcock notes. For example, University of Melbourne pathologist Kevin Barnham inhibited glycogen syn-thase kinase-3 (GSK-3) with a copper- bis (thiosemicar–bazone) complex that re-duced the abundance of hyperphosphory-lated tau and amyloid-β oligomers and also reversed cognitive deficits in mice ( Proc. Natl. Acad. Sci. USA 2009, 106, 381).

Columbia University pathologist Karen E. Duff and others have experimented with lithium as an inhibitor for GSK-3 in mice. Duff is also studying compounds that dis-rupt tau aggregation, such as cyanine dyes ( J. Med. Chem. 2009, 52, 3539).

Other dyes have been used as well. Claude M. Wischik, a molecular neuro-pathologist at the University of Ab-erdeen, in Scotland, has developed a form of methylene blue

that prevents tau ag-gregation and breaks up existing tau aggregates. Wischik, who in 1988 helped determine that tangles are made of tau, cofounded Singapore-based TauRx Therapeu-tics to further develop the methylene blue com-pound, dubbed Rember.

Other researchers studying methylene blue include Chad A. Dickey, an assistant professor of molecular medicine at the University of South Florida, in Tampa, who has been investigating its mechanism of action. In work with University of Michigan biological

chemist Jason E. Gestwicki, Dickey showed that the dye inhibits a tau-associated chaperone known as heat shock protein 70 (Hsp70) ( J. Neurosci. 2009, 29, 12079). The researchers are testing additional chaper-one inhibitors, including one based on a dihydropyrimidine scaffold.

Chaperone proteins help fold tau, con-trol its association with microtubules, and regulate its degradation. Tau itself is an unstructured, or “intrinsically disordered,” protein, Dickey explains. But interactions with chaperones and other biomolecules can confer specific conformations on tau, he believes. Hyperphosphorylation or trun-

cation of tau under disease conditions might alter these interactions, thereby changing tau’s folding behavior and degrada-tion. Furthermore, diseased tau might bind to normal tau and force it to adopt the diseased confor-mation, spreading tau pathology in a self-sustaining manner reminiscent of prion behavior, Dickey suggests.

Dickey and other researchers—includ-ing Petrucelli—are studying how to disrupt this destruc-tive sequence by

SCIENCE & TECHNOLOGY

TAUOPATHY HYPOTHESIS Biochemical cargo moves along microtubules within a neuron. Tau proteins (black) help

assemble and stabilize the microtubules, which consist of α- and β-tubulin dimers (yellow and red circles). In tauopathies, alterations in tau interfere with microtubule stabilization

and cargo transport. For example, addition of excess phosphate groups (green) by kinases detaches tau from microtubules—leading to their breakdown—and results in

accumulation of toxic tau aggregates, including paired helical filaments (PHF).

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AGGREGATE BUSTERS Methylene blue and a cyanine dye such as the one shown can break up tau aggregates.

39WWW.CEN-ONLINE.ORG MAY 3, 2010

manipulating chaperones including Hsp70 and Hsp90.

Compounds that help stabilize mi-crotubules when healthy tau isn’t readily available for this purpose have shown some promise in animal trials. One example is paclitaxel; another is nicotinamide. Besides enhancing microtubule stability, this form of vitamin B-3 reduces levels of hyperphos-phorylated tau and prevents memory loss in mice genetically engineered to develop Alz heimer’s, according to UC Irvine’s LaFerla ( J. Neurosci. 2008, 28, 11500). His Irvine colleague Steven S. Schreiber has be-gun a clinical trial of the compound.

ANOTHER APPROACH enlists the im-mune system to do battle with pathological tau. Sigurdsson’s team is testing two types of immunotherapy in mice genetically engi-neered to develop tau pathology as they age. In the “active immunization” method, the researchers inject the animals with small, hyperphosphorylated fragments of the tau molecule. The mouse immune system re-sponds to these antigens by producing an-tibodies against the hyperphosphorylated fragments. These regions are either absent or less prevalent in normal tau, so the anti-bodies “target the diseased molecules, not the normal ones,” Sigurdsson says.

Some of the antibodies move from the mouse’s bloodstream into its brain, because inflammation caused by neurodegeneration makes the blood-brain barrier leaky. Dis-eased neurons take up some of the antibod-ies, which bind to pathological aggregates of

hyperphosphorylated tau inside the cells. Once inside the neurons, Sigurdsson

thinks the antibodies promote the disas-sembly of tau aggregates by lysosomes. Lysosomes are cellular organelles loaded with enzymes that break down cellular components, such as proteins, that are defective or no longer needed. In an animal suffering from a tauopathy, the lysosomes are unable to keep up with the overwhelm-ing quantity of defective tau aggregates that need to be digested. Sigurdsson believes the tau antibodies counteract this problem by “facilitating the enzymatic degradation that takes place in the lysosomes.” The treat-ment prevents cognitive deterioration in mice ( Curr. Alz heimer Res. 2009, 6, 446).

Recent studies have suggested that tau may be secreted and taken up into neighbor-ing neurons, which could serve as a mecha-nism to spread tau pathology throughout the brain. In addition to degrading tau in-side neurons, the antibodies might also pro-mote clearance of extracellular tau, Sigur-dsson suggests. This action might prevent uptake of the diseased tau into neighboring cells and limit the spread of pathology.

Sigurdsson and colleagues are also pur-suing a “passive immunization” method in which they inject mice with monoclonal antibodies rather than antigens. They are assessing various regions of the diseased tau protein as targets for those antibodies.

Sigurdsson and Wilcock are both study-ing amyloid therapies in addition to tau treatments, believing that neither the amy-loid nor the tau camp has all the answers.

“If you can target both of these connected pathologies, you may get a better effect therapeutically than if you just target them individually,” Sigurdsson explains.

Last year, Wilcock reported results from a study of mice that develop amyloid depos-its, hyperphosphorylated and aggregated tau, and cognitive difficulties. On the basis of that study, Wilcock and her Duke col-league Carol A. Colton were the first to show that immunizing the mice against amyloid-β reduced both amyloid-β and tau pathology, halted neuron loss, and improved the ani-mals’ learning and memory capabilities ( J. Neurosci. 2009, 29, 7957). Wilcock cautions that the therapy causes limited brain hem-orrhaging, the consequences of which are unknown but must be addressed before the treatment can move forward.

The ultimate question, Wilcock says, is “What pathology do you target? If you tar-get tau, is that going to be enough to treat Alz heimer’s disease?”

Of course, tau-targeting therapies have relevance for “several tauopathies where you don’t have any amyloid-β pathology,” Sigurdsson notes. “And even though these are rarer, there is certainly treatment need-ed for these conditions as well.”

The goal of these researchers is to re-verse neurodegeneration, but they also believe that slowing the disease process is a worthy aim. “The progression into full-blown dementia takes several years,” Sigurdsson notes. “If you can maintain someone as mildly cognitively impaired, that is a great battle won already.” ■