The Tangled Neuron

Alzheimer's, Tangles and Tau

Summary: The tangles seen in Alzheimer’s brains are made up of a protein called tau. Tau is involved in Alzheimer’s and other neurodegenerative diseases, but scientists don’t yet understand its role. In the long run, understanding how tau contributes to brain cell death may help researchers develop new Alzheimer’s treatments.

More than a year after Dad died, I’m still trying to understand what caused his dementia and death. His primary diagnosis was cerebral amyloid angiopathy, but “it is likely that the presence of plaques and tangles contributed to his neurologic difficulties,” his autopsy consultation report says.

These plaques and tangles are signs of Alzheimer’s disease. A lot of Alzheimer’s research focuses on plaques and on beta amyloid, the protein that makes up those plaques. Several drugs aimed at preventing the build-up of beta amyloid are being tested. But what about tangles?

The tangles, also found in the brains of people with Pick’s disease and Parkinson’s, are made up of a protein called tau. Tau is found in “normal” brains, and helps stabilize the small tubes that are part of cells’ skeletons. These microtubules transport nutrients through the cells.

Much has been written about the disagreement between scientists who believe beta amyloid causes the brain degeneration seen in Alzheimer’s [“baptists”] and those who believe that tau is the culprit [“tauists”]. But it’s not really an either/or situation - we need to understand the role each protein plays and how they may work together.

Do Tangles Harm Brain Cells?

Were the tangles found in Dad’s brain really harmful to his brain cells? “We’re not certain,” says Dr. Lester Binder, Professor of Cell and Molecular Biology at Northwestern University. “It may be that they’re somewhat protective.”

Lester I Binder, Ph.D.

“Evidence indicates that tangles persist in some neurons for 25 to 35 years,” he says. “Eventually, these cells die, but perhaps not nearly as fast as some other non-tangle bearing neurons.”

In 2005, the results of two studies added to the evidence that tangles might not be causing cell death. Researchers at the University of Minnesota found that when they suppressed tau in mice bred to have human tau, memory improved and cell death stopped, but the tangles continued to form. Another study at the Albert Einstein College of Medicine showed that the presence of tangles in mouse brains didn’t always lead to cell death – instead, it appeared that cell death was caused by brain cells abnormally attempting to enter the cell cycle. The cell cycle is the process in which the chromosomes in a cell are replicated and the cell divides into two new cells. This re-entry into the cell cycle is not normal for adult brain cells, and has previously been linked to neurodegenerative diseases.

Do Changes to Tau Cause Cell Death?

If the tangles themselves don’t cause cell death, then what does? This “whodunit” mystery is more complicated than a game of Clue. In Clue, Mr. Boddy has been murdered, and players must consider six suspects, six possible murder weapons, and nine rooms in which the murder could have been committed. In Alzheimer’s, Mr. Brain Cell is dead, and scientists must consider hundreds of suspects, possible weapons and crime scenes. Those investigating tau are dealing with a lot of uncertainties. Possible tau-related weapons include:

- changes in the form of tau, perhaps when a phosphate gets added to the protein
- changes in the level of tau
- changes in the ratios of types of tau .

There’s uncertainty about who or what causes the changes in tau, too. The list of suspects includes:

- beta amyloid (the protein that makes up Alzheimer’s plaques)
- alpha-synuclein, another protein found in Parkinson’s disease and Alzheimer’s of the Lewy Body type
- impaired glucose metabolism
- a combination of genes plus exposure to toxic metals through diet causing oxidation.

Researchers are also trying to narrow down the location where tau-related cell death might start. Maybe death originates in the microtubules, which could be destabilized by changes in tau. Maybe it was in the cell cycle, but what was Mr. Brain Cell doing there anyway?

In the game of Alzheimer’s, players researching tau and tangles are a long way from winning.

Measuring Tau May Help with Diagnosis

Back in the real world, measuring levels of tau may be useful for diagnosing degenerative brain diseases. Along with levels of beta amyloid, levels of abnormal tau in spinal fluid might predict conversion from mild cognitive impairment to Alzheimer’s or help diagnose Alzheimer’s. Innogenetics, a company based in Belgium, has developed a test to measure tau levels.

Using PET (Positron Emission Tomography) scans and other imaging techniques to map tau in the brain may also help with diagnosis in the future. In late 2006, researchers at UCLA reported that when they injected a substance called FDDNP into 83 people with memory problems, it stuck to both plaques and tangles and could be seen during a PET scan. [If you really want to know, FDDNP stands for fluorescent probe 2-{1-[6-(dimethylamino)-2-naphthyl]ethylidene}malononitrile.] The amount of plaques and tangles shown by FDDNP correlated with the severity of cognitive impairment. The study authors concluded this scan could be used as a tool to differentiate among people with normal memory, those with mild cognitive impairment, and those with Alzheimer’s.

In the future, measuring and mapping tau before and after a treatment may be one indicator of whether that treatment is working.

Potential Alzheimer’s Treatments Targeting Tau

As with beta amyloid, scientists are looking for ways to prevent or treat Alzheimer’s by tinkering with tau. Dr. Binder and his colleagues have shown that an enzyme called puromycin-sensitive aminopeptidase degrades tau in the lab. Scientists at the University of British Columbia are studying how thrombin, a protein involved in blood coagulation, degrades tau, and University of California researchers have found that removing beta amyloid may clear certain forms of tau.

Besides eliminating or reducing tau, other approaches include:

- preventing changes to tau (using lithium or other substances)
- protecting the microtubules in brain cells (using an enzyme called Pin1, the cancer drug Taxol or a compound called NAP, developed by Allon Therapeutics Inc. and now in early clinical trials).

Some scientists think tinkering with beta amyloid could do more harm than good, and tau is no different. I asked Dr. Binder if potential therapies could remove too much tau, since the protein is needed for normal brain function. “This isn’t known as yet,” he says. “Tau knockout mice [bred to have no tau] survive, but it’s not clear how normal they are.”

If removing tau might be dangerous, would it be better to try to regulate the sequence of events leading to changes in tau, or the formation of tangles, rather than remove tau or tangles?

“Perhaps,” says Dr. Binder. “Unfortunately, we are somewhat uninhibited by facts here,” he notes, referring to all the uncertainties about tau. He is sure, however, that investigating tau is worthwhile. “Tau is certainly part of the problem in Alzheimer’s disease and certain inherited frontal lobe dementias,” he says, “and the link between amyloid and tau is currently being explored by both baptists and tauists alike.”

I put Dad’s autopsy report back in the file cabinet for a while. It may be a few more years before I can understand what the tangles were doing in my father’s brain.

Alzheimer's Plaque-Buster?

For years, Dad had a night job. By day, he ran our family’s retail lumberyard. By night, he ran a mouse relocation program. Our house was full of mice. They scampered through the attic and ran down plumbing and electrical ducts, nabbing cotton balls and Kleenex to build their nests. They got into our food, and left droppings along the baseboard.

The situation came to a head when my mother went into the kitchen to get a plastic bag full of garbage she had left on the counter. “There was a mouse sitting inside the bag, stuffing his face,” she says. She called Dad into the kitchen. He carefully closed the bag, carried it outside, and let the mouse go.

That night, my father brought home a small Havahart trap, baited it with peanut butter, and set it in the laundry room. A quivering brown mouse was in the cage the next morning. On his way to work, Dad drove his passenger down a dirt road, and released him in the woods.

Dad reset the trap that night, and another mouse was there in the morning. This went on for months – almost every morning, another mouse. And almost every morning, Dad would head down the dirt road to his mouse drop-off spot.

“They’ve posted a sign,” he joked, “free peanut butter and a ride.” My parents kept a tally - after four months, Dad had transported 123 mice. They seemed to be multiplying faster than he could clear them out.

Breaking Down Beta Amyloid

Scientists think the same thing may be happening with beta amyloid, the protein that makes up the plaques found in Alzheimer’s. The protein is constantly produced by the body, then cleared from the brain. It may be that in people with Alzheimer’s, the beta amyloid is building up faster than it can be eliminated, and the excess protein is toxic to brain cells.

Researchers have found several enzymes that break down beta amyloid, including insulin degrading enzyme (IDE). Increasing the amount of these substances in the body could help speed up the elimination of excess beta amyloid.

Dr. Jin-Moo Lee, Assistant Professor of Neurology at Washington University School of Medicine in St. Louis has shown that another enzyme called matrix metalloprotease-9 (MMP-9) also degrades beta amyloid. Dr. Lee found that MMP-9 is able to break down the fibrils that make up the plaques found in Alzheimer’s. MMP-9 and other enzymes break down a free-floating kind of beta amyloid that hasn’t formed into plaques. But in Dr. Lee’s lab, the other enzymes didn’t seem to degrade fibrils the way MMP-9 did. These results suggest that MMP-9, already found in the body, may be helpful in clearing plaques from the brain.

Jin-Moo Lee, M.D., Ph.D.

There’s more evidence that the enzyme may help regulate beta amyloid levels. Dr. Lee and his colleagues found that turning off the gene for MMP-9 in mice increased the levels of beta amyloid in their brains.

MMP-9’s Role In Other Diseases

Harnessing MMP-9 to break down beta amyloid will be a delicate task. High levels of the enzyme are associated with cancer and arthritis. Even worse in terms of dementia, Dr. Lee has shown that high levels of MMP-9 near blood vessel walls in the brain are associated with cerebral amyloid angiopathy (CAA). In CAA, beta amyloid is deposited on blood vessel walls in the brain. The walls then thicken, harden and crack, allowing blood to leak out into the surrounding tissue. My father’s autopsy showed severe CAA, and it’s likely that the disease and resulting microbleeds caused his dementia. He died last year of a massive hemorrhagic stroke, probably due to CAA.

So MMP-9 may clear Alzheimer’s plaques, but may also be involved in CAA and brain hemorrhages. Is MMP-9 good or bad for the brain?

“MMP-9 is neither ‘good’ nor ‘bad,’ Dr. Lee says, “but may have different responses and activities in different cells and different locations. We have hypothesized that MMP-9 may play a role in weakening the vessel wall in CAA (though this is far from proven)…. We have shown that MMP-9 and -2 can degrade Abeta [beta amyloid] in the brain. These are two different locations and two different activities.”

The fact that increased levels of MMP-9 have been found in the brains of ischemic stroke patients provided a clue to MMP-9’s role in brain hemorrhages. “MMP-9 is upregulated after ischemic stroke,” Dr. Lee explains, “and likely plays a role in converting an ischemic stroke into a hemorrhagic stroke (by weakening the vessel wall). This is why it is so intriguing that MMP-9 is upregulated in CAA vessels, with the thought that CAA vessels may have increased MMP-9 which might eventually lead to weakened vessels.”

Dr. Lee is working to confirm the role of MMP-9 in CAA. He has a study underway to see if lowering MMP-9 levels in mice with CAA reduces the frequency of brain hemorrhages. “This data will be far more convincing,” he says.

More Research Needed

Adding to the uncertainty of MMP-9’s role in dementia, it’s not clear what effect breaking down the beta amyloid fibrils will have. “Again, this may be "good" or "bad," Dr. Lee explains. “Recent reports suggest that smaller aggregates of Abeta (oligomers) may be toxic or inhibit neurotransmission. It is possible that MMP-9 may break down fibrils into these smaller aggregates, which might be even more toxic than fibrils. On the other hand, it is possible that MMP-9 could degrade both fibrils and oligomers rendering them non-toxic. We are currently investigating this.”

It’s also not clear whether drugs designed to increase or decrease levels of MMP-9 could stop brain degeneration and dementia, or what any side effects would be. One possibility for therapy stems from the fact that MMP-9, like many other enzymes, requires the presence of zinc to work. So in theory, reducing the amount of zinc in the body via chelation therapy might inactivate MMP-9 and reduce damage to blood vessel walls. It’s unclear what this might do to the beta amyloid plaques in other areas of the brain, though, and there could be severe side effects. “Removing zinc would likely be detrimental to other systems,” says Dr. Lee.

The connection of MMP-9 to both Alzheimer’s and CAA is intriguing, but still murky. More research is needed before any treatments can be developed. “I think we are far from therapies at this point,” Dr. Lee says. “One must remember that at this level of research, we are trying to understand molecular mechanisms, and we are somewhat removed from therapies. However, our goal is to identify potential targets for the development of therapies. It’s too early to say whether MMP-9 will provide us with viable targets, but therapies to ameliorate disease are always on our minds.”