tauopathies

How protein tangles accumulate in the brain and cause neurological disorders

A new Sanford Burnham Prebys Medical Discovery Institute (SBP) study takes a step forward in understanding how similar, yet genetically unrelated neurodegenerative diseases, such as Alzheimer’s disease, frontal temporal dementia, and progressive supranuclear palsy (PSP) are caused by the protein tau. The findings, published in Neuron, create new opportunities to target this key protein that leads to the brain lesions found in patients with impaired motor functions and dementia.

Our research shows how the abundance of a protein called appoptosin increases tau aggregates called tangles, which are toxic to the brain and lead to the progressive deterioration of the central nervous system,” said Huaxi Xu, professor in the Degenerative Diseases Program at SBP. “By understanding how appoptosin drives this process, we can now look at ways to inhibit key triggering points and potentially slow the progression of this class of neurodegenerative diseases which are collectively known as tauopathies.”

What are tauopathies?
Tauopathies are neurodegenerative diseases that are signified by the presence of irregular tangle-like clumps of the protein tau that appear in the brain and accumulate during as the disease progresses. Because tau tangles appear in numerous diseases such as Alzheimer’s and PSP, it seemed likely that tau could be a key factor in causing neuron and brain malfunction in these diseases. However, how diseases such as PSP are triggered, and whether similar causes could generate these tau protein tangles were unknown.

Tau is a protein that maintains the integrity of long hollow tube structures called microtubules, which are major structural elements of cells. In neurons, microtubules form long extensions called axons where signals are rapidly transported over long distances when neurons communicate with each other. When tau becomes abnormally modified by hyperphosphorylation, or cleaved by the enzyme caspase-3, which itself can also facilitate its hyperphosphorylation, it loses its biological activity and goes through conformational changes that allow the protein to accumulate and form tangles.

Because it was largely unknown how caspase-3 is triggered to induce tau aggregation in tangles, determining the sequence of events that lead to tau cleavage and aggregation is one of the most important goals for the prevention and treatment of tauopathies.

Key findings of the paper

The new paper highlights a novel role for appoptosin in neurological tauopathy disorders such as PSP. PSP is a neurological disease with tau brain aggregates, where patients experience serious problems with balance, eye movement and thinking. Until now, genetic and biological triggers for PSP were unknown. By examining patients with PSP, it became apparent that variation in the DNA sequence of a single nucleotide (a SNP) was associated with the disease and correlated with elevated levels of appoptosin that increased caspase-mediated tau cleavage, tau aggregation, and synaptic dysfunction.

Neurodegenerative triggering factors appoptosin and caspase-3 cleaved tau were also found to be overabundant or over-activated in brain samples of patients with Alzheimer’s disease and frontotemporal dementia, supporting the importance of their contribution to these neurodegenerative disorders.

“A better understanding of the mechanisms that cause neurofibrillary tangles is of clinical importance for developing therapeutic strategies to prevent and treat tauopathies,” added Xu. “Our findings suggest that appoptosin and/or caspase-3 may be potential targets in the treatment of these neurodegenerative diseases.”

(Image caption: Given an opportunity to spread in cells, prion-like proteins taken from the brains of patients with (from top) Alzheimer’s disease, corticobasal degeneration and Pick’s disease form distinctly shaped clumps (green in this image) in different parts of the cells. Credit: David W. Sanders)

Alzheimer’s disease, other conditions linked to prion-like proteins

A new theory about disorders that attack the brain and spinal column has received a significant boost from scientists at Washington University School of Medicine in St. Louis.

The theory attributes these disorders to proteins that act like prions, which are copies of a normal protein that have been corrupted in ways that cause diseases. Scientists previously thought that only one particular protein could be corrupted in this fashion, but researchers in the laboratory of Marc Diamond, MD, report that another protein linked to Alzheimer’s disease and many other neurodegenerative conditions also behaves very much like a prion.

The findings appear online May 22 in Neuron.

Diamond’s lab found that the protein, known as tau, could be corrupted in different ways, and that these different forms of corruption — known as strains — were linked to distinct forms of damage to the brain.

“If we think of these different tau strains as different pathogens, then we can begin to describe many human disorders linked to tau based on the strains that underlie them,” said senior author Diamond, the David Clayson Professor of Neurology. “This may mean that certain antibodies or drugs, for example, will work better against certain disorders than others.”

The study was led by co-first authors David Sanders and Sarah Kaufman, who are graduate students.

Prions are composed of normal proteins that have folded into an abnormal shape. They aren’t alive, but their effects can be similar to infectious microbes such as bacteria or viruses. Their unusual structure lets prions replicate themselves through a kind of molecular peer pressure: When a prion interacts with identical but normally folded proteins, it can cause these proteins to become prions, which are small aggregates, or clumps, that can spread from cell to cell.

Prions first came to popular attention in the 1990s with the emergence of mad cow disease, a disorder that destroys the brains of cattle. Scientists linked a few cases of a similar condition in people to consumption of meat from infected cows. Researchers eventually determined that the disorder was caused by a distinct strain of prions made by the sickened cattle.

Scientists had suspected that prion-like forms of a protein called alpha-synuclein contribute to Parkinson’s disease and other conditions, and prion-like versions of proteins known as SOD1 and TDP43 may cause amyotrophic lateral sclerosis, commonly known as Lou Gehrig’s disease.

Scientists also had identified tau clumps in 25 different neurodegenerative disorders, known collectively as tauopathies. This hinted at potential prion-like behavior on the part of tau. In 2009, Diamond’s group found that tau misfolds into several different shapes in a test tube.

“When we infected a cell with one of these misshapen copies of tau and allowed the cell to reproduce, the daughter cells contained copies of tau misfolded in the same fashion as the parent cell,” Diamond said. “Further, if we extracted the tau from an affected cell, we could reintroduce it to a naïve cell, where it would recreate the same aggregate shape. This proves that each of these differently shaped copies of the tau protein can form stable prion strains, like a virus or a bacteria, that can be passed on indefinitely.”

Diamond used the tau prions made in cells to infect mouse brains, showing that differently shaped strains caused different levels of brain damage. He isolated the prions from the mice, grew them in cell culture, and then infected other mice. Throughout these transfers, each particular prion strain continued to be misfolded in the same shape and to cause damage in the same fashion.

Finally, the researchers examined clumps of tau from the brains of 28 patients after they died. Each of the patients was known to have one of five forms of tauopathy.

“Each disease had a unique tau prion strain or combination of strains associated with it,” he said. “For example, we isolated the same tau prion strain from nearly every patient with Alzheimer’s disease we examined.”

Brain samples from patients with the progressive neurological disorderscorticobasal degeneration and Pick’s disease also typically had the same tau prion strains or mixtures of strains.

Diamond and others now are working to find a way to isolate tau prions non-invasively from individuals for diagnostic purposes.

Options for stopping prions include monoclonal antibodies, which could label prions for inactivation or immune system attack and removal (described in a paper by Diamond and David Holtzman, MD, Chair of Neurology (Neuron, 2013)). Diamond and others also are developing ways to block tau prion movement between cells and to stop cells from making new copies of the prion proteins.

Disease in a Dish

Scientists use latest stem cell and gene-editing techniques to generate neurons in a dish, and reveal new clues behind deadly diseases of the brain

There is no easy way to study diseases of the brain. Extracting neurons from a living patient is both difficult and risky, while examining a patient’s brain post-mortem usually only reveals the disease’s final stages. And animal models, while incredibly informative, have frequently fallen short during the crucial drug-development stage of research. The result: we are woefully unprepared to fight—and win—the war against this class of diseases.

But scientists at the Gladstone Institutes and the University of California, San Francisco (UCSF) are taking a potentially more powerful approach: an advanced stem-cell technique that creates a human model of degenerative disease in a dish.

Using this model, the team uncovered a molecular process that causes neurons to degenerate, a hallmark sign of conditions such as Alzheimer’s disease and frontotemporal dementia (FTD). The results, published in the latest issue of Stem Cell Reports, offer fresh ammunition in the continued battle against these and other deadly neurodegenerative disorders.

The research team, led by Gladstone Investigator Yadong Huang, MD, PhD, identified an important mechanism behind tauopathies. A group of disorders that includes both Alzheimer’s and FTD, tauopathies are characterized by the abnormal accumulation of the protein Tau in neurons. This buildup is thought to contribute to the degeneration of these neurons over time, leading to debilitating symptoms such as dementia and memory loss. But while this notion has been around for a long time, the underlying processes have largely remained unclear.

“So much about the mechanisms that cause tauopathies is a mystery, in part because traditional approaches—such as post-mortem brain analysis and animal models—give an incomplete picture,” explained Dr. Huang. “But by using the latest stem-cell technology, we generated human neurons in a dish that exhibited the same pattern of cell degeneration and death that occurs inside a patient’s brain. Studying these models allowed us to see for the first time how a specific genetic mutation may kick start the tauopathy process.”

Other scientists recently discovered that the Tau mutation in question could increase a person’s risk of developing different tauopathies, including Alzheimer’s or FTD. So the research team, in collaboration with Bruce Miller, MD, who directs the UCSF Memory and Aging Center and who provided skin cells from a patient with this mutation, transformed these cells into induced pluripotent stem cells, or iPS cells. This technique, pioneered by Gladstone Investigator and 2012 Nobel Laureate Shinya Yamanaka, MD, PhD, allows scientists to reprogram adult skin cells into cells that are virtually identical to stem cells. These stem cells can then develop into almost any cell in the body.

The team combined this method with a cutting-edge gene-editing technique that essentially eliminated the Tau mutation in some of the iPS cells. The result was a system that allowed the team to compare neurons that had the mutation to those that did not.

“Our approach allowed us to grow human neurons in a dish that contained the exact same mutation as the neurons in the brain of the patient,” explained first author Helen Fong, PhD, who is also a California Institute for Regenerative Medicine postdoctoral scholar. “By comparing these diseased neurons with the ‘genetically corrected’ healthy neurons, we could see—cell by cell—how the Tau mutation leads to the abnormal build up of Tau and, over time, neuronal degeneration and death.”

“Tau’s main functions include keeping the skeletal structure of individual neurons intact and regulating neuronal activity,” said Dr. Huang. “But our research showed that the Tau produced by neurons from people with the Tau mutation is different; so it is red-flagged by the cell and targeted for destruction. However, instead of being flushed out, Tau gets chopped into pieces. These potentially toxic fragments accumulate over time and may in fact cause the neuron to degenerate and die.”

But by correcting the Tau mutation, the team effectively removed Tau’s red flag. The protein remained in one piece, the abnormal buildup ceased and the neurons remained healthy. Ongoing studies aim to determine whether the abnormal fragmentation and buildup of mutant tau is really the main cause of the neuronal death and, if so, how to block it.

Finding a way to block this toxic buildup of tau fragments has been a key focus of drug development—but has thus far been unsuccessful. But Dr. Huang and his colleagues are optimistic that their approach could be exactly what researchers need to fight back against deadly tauopathies.

“These findings not only offer a glimpse into how these powerful new models can shed light on mechanisms of disease” said Dr. Miller, “They may also prove invaluable for screening potential drugs that could be developed into better treatments for Alzheimer’s disease, FTD and related conditions.”