How Prion Protein Damages Brain Cells
Scientists at the National Institutes of Health have gained a major insight into how the rogue protein responsible for mad cow disease and related neurological illnesses destroys healthy brain tissue.
"This advance sets the stage for future efforts to develop potential treatments for prion diseases or perhaps to prevent them from occurring." said Duane Alexander, M.D., Director of NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), where the study was conducted.
The researchers discovered that the protein responsible for these disorders, known as prion protein (PrP), can sometimes wind up in the wrong part of a cell. When this happens, PrP binds to Mahogunin, a protein believed to be essential to the survival of some brain cells. This binding deprives cells in parts of the brain of functional Mahogunin, causing them to die eventually. The scientists believe this sequence of events is an important contributor to the characteristic neurodegeneration of these diseases.
The findings were published in the current issue of the journal Cell. The study was conducted by Oishee Chakrabarti, Ph.D. and Ramanujan S. Hegde, M.D., Ph.D., of the NICHD Cell Biology and Metabolism Program.
Central to prion diseases like mad cow disease and to many other diseases is the phenomenon known as protein misfolding, Dr. Hegde explained. Proteins are made up of long chains of molecules known as amino acids. When proteins are created, they must be carefully folded into distinct configurations. The process of protein folding is analogous to origami, where a sheet of paper is folded into intricate shapes. Upon correct folding, proteins are transported to specific locations within cells where they can perform their various functions. However, the protein chains sometimes misfold. When this happens, the incorrectly folded protein takes the wrong shape, cannot function properly, and as a consequence, is sometimes relegated to a different part of the cell.
In the case of prion diseases, the culprit protein that misfolds and causes brain cell damage is PrP. Normally, PrP is found on the surface of many cells in the body, including in the brain. However, the normal folding and distribution of PrP can go wrong. If a rogue misfolded version of PrP enters the body, it can sometimes bind to the normal PrP and "convert" it into the misfolded form.
This conversion process is what causes mad cow disease, also known as bovine spongiform encephalopathy. Feed prepared from cattle tissue containing an abnormally folded form of PrP can infect cows. In very rare instances, people eating meat from infected cows are thought to have contracted a similar illness called variant Creutzfeld Jacob disease (vCJD). In other human disorders, genetic errors cause other abnormal forms of PrP to be produced.
"The protein conversion process has been well studied," Dr. Hegde said. "But the focus of our laboratory has been on how — and why — abnormal forms of PrP cause cellular damage."
To investigate this problem, Dr. Hegde’s team has been studying exactly how, when, and where the cell produces abnormal forms of PrP. They had found that many of the abnormal forms of PrP were located in the wrong part of the cell. Rather than being on the cell’s surface, some PrP is exposed to the cytoplasm, the gelatinous interior of the cell. Moreover, several studies from Dr. Hegde’s group and others showed that when too much of a cell’s PrP is exposed to the cytoplasm in laboratory mice, they develop brain deterioration.
"The sum of these discoveries provided us with a key insight," Dr. Hegde said. "We realized that in at least some cases, PrP might be inflicting its damage by interfering with something in the cytoplasm."
In the current study, Drs. Chakrabarti and Hegde sought to determine what went wrong when PrP was inappropriately exposed to the cytoplasm. Their next clue came from a strain of mice with dark mahogany-colored fur. Although these mice develop normally at first, parts of their nervous systems deteriorate with age. Upon autopsy, their brains are riddled with tiny holes, and have the same spongy appearance as the brains of people and animals that died of prion diseases. The gene that is defective in this strain of mice is named Mahogunin.
"The similarity in brain pathology between the Mahogunin mutant mice and that seen in prion diseases suggested to us that there might be a connection," Dr. Hegde said.
To investigate this possible connection, the researchers first analyzed PrP and Mahogunin in cells growing in a laboratory dish. When the researchers introduced altered forms of PrP into the cytoplasm of cells, they saw that Mahogunin molecules in the cytoplasm bound to the PrP, forming clusters. This clustering led to damage in the cell that was very similar to the damage occurring when cells are deprived of Mahogunin.
The researchers found that this damage did not occur in the cell cultures if PrP was confined to the surface of the cell, if the cells were provided with additional Mahogunin, or if PrP was prevented from binding to Mahogunin.
The researchers then studied mice with a laboratory induced version of a human hereditary prion disorder called GSS, or Gerstmann-Straussler-Scheinker Syndrome. This extremely rare disease causes progressive neurological deterioration, typically leading to death between age 40 to 60. Dr. Hegde explained that some GSS mutations result in a form of PrP that comes in direct contact with the cytoplasm. In mice that contain one of these mutations, the researchers discovered that cells in parts of the brain were depleted of Mahogunin. The researchers did not see this depletion if PrP was engineered to avoid the cytoplasm.
The findings, Dr. Hedge said, strongly suggest that altered forms of PrP interfere with Mahogunin to cause some of the neurologic damage that occurs in prion diseases.
"PrP probably interferes with other proteins too," Dr. Hegde said. "But our findings strongly suggest that the loss of Mahogunin is an important factor."
An understanding of how PrP interacts with Mahogunin sets the stage for additional studies that may find ways to prevent PrP from entering the cytoplasm, or to replace Mahogunin that has been depleted.