(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.

What Makes Memories Last?

Prions can be notoriously destructive, spurring proteins to misfold and interfere with cellular function as they spread without control. New research, published in the open access journal PLOS Biology on February 11, 2014, from scientists at the Stowers Institute for Medical Research reveals that certain prion-like proteins, however, can be precisely controlled so that they are generated only in a specific time and place. These prion-like proteins are not involved in disease processes; rather, they are essential for creating and maintaining long-term memories.

“This protein is not toxic; it’s important for memory to persist,” says Stowers researcher Kausik Si, Ph.D., who led the study. To ensure that long-lasting memories are created only in the appropriate neural circuits, Si explains, the protein must be tightly regulated so that it adopts its prion-like form only in response to specific stimuli. He and his colleagues report on the biochemical changes that make that precision possible.

Si’s lab is focused on finding the molecular alterations that encode a memory in specific neurons as it endures for the days, months, or years—even as the cells’ proteins are degraded and renewed. Increasingly, their research is pointing toward prion-like proteins as critical regulators of long-term memory.

In 2012, Si’s group demonstrated that prion formation in nerve cells is essential for the persistence of long-term memory in fruit flies. Prions are a fitting candidate for this job because their conversion is self-sustaining: once a prion-forming protein has shifted into its prion shape, additional proteins continue to convert without any additional stimulus.

Si’s team found that in fruit flies, the prion-forming protein Orb2 is necessary for memories to persist. Flies that produce a mutated version of Orb2 that is unable to form prions learn new behaviors, but their memories are short-lived. “Beyond a day, the memories become unstable. By three days, the memory has completely disappeared,” Si explains.

In the new study, Si wanted to find out how this process could be controlled so that memories form at the right time. “We know that all experiences do not form long-term memory—somehow the nervous system has a way to discriminate. So if prion-formation is the biochemical basis of memory, it must be regulated.” Si says. “But prion formation appears to be random for all the cases we know of so far.”

Si and his colleagues knew that Orb2 existed in two forms—Orb2A and Orb2B. Orb2B is widespread throughout the fruit fly’s nervous system, but Orb2A appears only in a few neurons, at extremely low concentrations. What’s more, once it is produced, Orb2A quickly falls apart; the protein has a half-life of only about an hour.

“When Orb2A binds to the more abundant form, it triggers conversion to the prion state, acting as a seed for the conversion. Once conversion begins, it is a self-sustaining process; additional Orb2 continues to convert to the prion state, with or without Orb2A. By altering the abundance of the Orb2A seed”, Si says, “cells might regulate where, when, and how the conversion process is engaged”. But how do nerve cells control the abundance of the Orb2A seed?

Their experiments revealed that when a protein called TOB associates with Orb2A , it becomes much more stable, with a new half-life of 24 hours. This step increases the prevalence of the prion-like state and explains how Orb2’s conversion to the prion state can be confined in both time and space.

The findings raise a host of new questions for Si, who now wants to understand what happens when Orb2 enters its prion-like state, as well as where in the brain the process occurs. While unraveling these mechanisms will likely be more accessible in the fruit fly than in more complex organisms, Si points out that proteins related to Orb2 and TOB have also been found in the brains of mice and humans. He has already shown that in the sea snail Aplysia, conversion to a prion-like state facilitates long-term change in synaptic strength. “This basic mechanism appears to be conserved across species,” he notes.

The flexible tail of the prion protein poisons brain cells

For decades, there has been no answer to the question of why the altered prion protein is poisonous to brain cells. Neuropathologists from the University of Zurich and University Hospital Zurich have now shown that it is the flexible tail of the prion protein that triggers cell death. These findings have far-reaching consequences: only those antibodies that target the tail of the prion protein are suitable as potential drugs for combating prion diseases.

Prion proteins are the infectious pathogens that cause Mad Cow Disease and Creutzfeldt-Jakob disease. They occur when a normal prion protein becomes deformed and clumped. The naturally occurring prion protein is harmless and can be found in most organisms. In humans, it is found in our brain cell membrane. By contrast, the abnormally deformed prion protein is poisonous for the brain cells. Adriano Aguzzi, Professor of Neuropathology at the University of Zurich and University Hospital Zurich, has spent many years exploring why this deformation is poisonous. Aguzzi’s team has now discovered that the prion protein has a kind of «switch» that controls its toxicity. This switch covers a tiny area on the surface of the protein. If another molecule, for example an antibody, touches this switch, a lethal mechanism is triggered that can lead to very fast cell death.

Flexible tail induces cell death

In the current edition of «Nature», the scientists demonstrate that the prion protein molecule comprises two functionally distinct parts: a globular domain, which is tethered to the cell membrane, and a long and unstructured tail. Under normal conditions, this tail is very important in order to maintain the functioning of nerve cells. By contrast, in the case of a prion infection the pathogenic prion protein interacts with the globular part and the tail causes cell death – this is the hypothesis put forward by the researchers.

Aguzzi and his team tested this by generating mimetic antibodies in tissue sections from the cerebellum of mice which have a similar toxicity to that of a prion infection. The researchers found that these antibodies tripped the switch of the prion protein. «Prion proteins with a trimmed version of the flexible tail can, however, no longer damage the brain cells, even if their switch has been recognized by antibodies», explains Adriano Aguzzi. «This flexible tail is responsible for causing cell death.» If the tail is bound and made inaccessible using a further antibody, activation of the switch can likewise no longer trigger cell death.

«Our discovery has far-reaching consequences for understanding prion diseases», says Aguzzi. The findings reveal that only those antibodies that target the prion protein tail are suitable for use as potential drugs. By contrast, antibodies that trip the switch of the prion are very harmful and dangerous.

The good side of the prion: A molecule that is not only dangerous, but can help the brain grow

A few years ago it was found that certain proteins, the prions, when defective are dangerous, as they are involved in neurodegenerative syndromes such as the Creutzfeldt-Jakob and the Alzheimer diseases. But now research is showing their good side, too: when performing well, prions may be crucial in the development of the brain during childhood, as observed by a study carried out by a team of neuroscientists at Trieste’s SISSA which appeared yesterday in the Journal of Neuroscience.

Doctor Jekyll and Mr. Hyde: the metaphor of the good man who hides an evil side suits well the prion (PrPC in its physiological cellular form), a protein which abounds in our brain. Unlike Doctor Jekyll, the prion was at first considered for its upsetting properties: if the molecule abnormally folds over itself it unfortunately plays a crucial role in neurodegenerative processes that lead to dreadful syndromes such as the mad cow disease.

Prions, however, in their normal form abound in synapses, the contact points where the nervous signal is passed from a neuron to the next. Such protein relatively abounds in the brain of very young children, and this is the reason why scientists have assumed it may play a role in the
nervous system development, and in particular in neurogenesis, in the development of new synaptic connections and in plasticity.

More in detail

Maddalena Caiati, Victoria Safiulina, Sudhir Sivakumaran, Giuseppe Legname, Enrico Cherubini, all researchers at SISSA, and Giorgia Fattorini of the Università Politecnica delle Marche have verified at the molecular level the effects of PrPC on the cell plasticity of the hippocampus, a brain structure which has important functions related to memory. Maddalena Caiati and her colleagues have demonstrated that PrPC controls synaptic plasticity (the growth capacity of the nervous tissue) through a transduction pathway which involves also another protein, the protein kinase A enzyme (PKA). The recently published research is only the starting point. As for the future, it will be interesting to get a closer look at the role played by the prion protein in the development of neuronal circuits both under physiological and pathologic conditions in neurodegenerative diseases.