HOW PROTEIN MISFOLDING CAN CAUSE DISEASE
The functional product of a gene is a protein. To get from DNA sequence to a protein, the DNA sequence is transcribed into messenger RNA (mRNA), the mRNA is then translated into a linear amino acid sequence, or polypeptide, and then the amino acids comprising the polypeptide interact with each other to form a folded protein. Protein structure has a large impact on the function of a protein, where the ultimate structure is determined by the underlying amino acid sequence. Thus, any changes in the DNA or RNA sequence used to generate the amino acid sequence of a protein can have dramatic effects on protein function. It is well known that many illnesses, including cancers, genetic diseases and infectious diseases, can result from mutations in a gene sequence. Amyloids are protein aggregates that cannot be dissolved and have specific structural characteristics. Amyloids result from inappropriate protein folding (misfolding), often due to an underlying mutation. The altered structure causes the proteins to interact and propagate the misfolded state. Depending on the organism and the specific misfolded protein, these amyloids can have potentially positive or negative effects on cell survival and health.
Protein misfolding, also called proteinopathy, can result in many different types of disease. Prions, are a specific type of misfolded protein, that are “infectious”. The most famous prions are those that can cause infectious encephalopathies, or degenerative brain diseases in humans and animals. Prions are responsible for Scrapie in sheep and goats, and bovine spongiform encephalopathy, also known as mad cow-disease in cattle, or Creutzfeldt–Jakob disease in humans. Naturally occurring prions have also been found in yeast. In yeast, proteins can switch back and forth between the prion, or misfolded protein state, and the normal protein state. Yeast prions can be passed between yeast cells, and are heritable, passing from one generation to the next. In contrast to the prions which cause encephalopathies, it is thought that prions in yeast could be an evolutionary advantage beneficial for survival. Several other well-known neurological diseases, such as Huntington’s, Alzheimer’s, and Parkinson’s disease have also been associated with proteinopathy. Thus, protein misfolding can result in several different types of diseases, including diseases that are infectious, chronic, genetic, degenerative, and almost always debilitating. While protein misfolding has been associated with a number of diseases, the underlying mechanism of how misfolded proteins cause these diseases remains unclear. Many of these diseases are very difficult to diagnose, have no cure, and effective treatment options are lacking. Understanding what genetic mutations cause misfolding, and how misfolded proteins interact with the host and the host immune system is critical to developing better prevention, detection and treatment methods for these types of diseases.
COMMON FUND PROTEIN MISFOLDING RESEARCH
Several NIH Common Fund High Risk-High Reward (HRHR) Awardees are performing cutting-edge research in their laboratories to understand how misfolded proteins cause disease. These HRHR researchers are investigating both the beneficial and negative effects of protein misfolding in many different model organisms. This research could lead to a better understanding of how we can treat and prevent illnesses associated with misfolded proteins.
RANDALL HALFMANN, PH.D., UT SOUTHWESTERN MEDICAL CENTER
Yeast have naturally occurring prions that are thought to play both beneficial and harmful roles. Dr. Randal Halfmann, a 2011 Early Independence Awardee, and his colleagues recently published a paper examining how yeast prions act in response to environmental conditions contributing to yeast cell fate. This research shows how yeast cells can switch between the prion form and the non-prion form of a specific yeast protein, transcriptional repressor Mot3, in response to food sources and stress. Yeast are involved in the making of wine which involves a process called fermentation, where the yeast convert sugar into a specific type of alcohol, ethanol. Carbon dioxide is also created during this process. Following fermentation, there is little sugar remaining in the environment, but a high concentration of ethanol which causes stress to the yeast. Under these stressful conditions, the yeast turn on the prion form of Mot3 [MOT3+], which allows the yeast to act as a multicellular complex rather than as individual cells, in order to breakdown the ethanol and protect the complex as a whole. Breaking down the alcohol, results in the reduction of oxygen. In this reduced oxygen environment the yeast turn off the prion form of Mot3 and the yeast cells return to acting as individual cells. Understanding how yeast are able to turn prions on and off, and how these prions are beneficial to yeast survival could lead a better understanding of how therapies might be developed to control and treat disease associated with misfolded proteins in human and animals.
Dr. Halfmann’s NIH Director’s Early Independence Award Project Abstract
UT Southwestern Medical Center Press Release on Halfmann’s recent work
Watch Dr. Halfmann describe this research here
D. L. Holmes, A. K. Lancaster, S. Lindquist, R. Halfmann, Heritable remodeling of yeast multicellularity by an environmentally responsive prion. Cell 153, 153 (2013).
JAMES SHORTER, PH.D., UNIVERSITY OF PENNSYLVANIA, AND AARON GITLER, PH.D., STANFORD UNIVERSITY
New Innovators, Drs. James Shorter and Aaron D. Gitler, have been investigating the effects of genetic mutations in prion-like domains (PrLDs), or areas of genes that have sequence similarities to prions, in specific RNA-binding proteins (hnRNPs). They have found that specific mutations in two hnRNP genes result in misfolded self-replicating “prion-like” protein aggregates that cause degenerative neurological diseases such as multisystem proteinopathy (MLS), and amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease. These authors suggest that there are around 250 human proteins that contain PrLDs that could serve as a starting point to screen for potential disease causing candidates.
Dr. Aaron D. Gitler’s NIH Director’s New Innovator Award Project Abstract
Dr. James Shorter’s NIH Director’s New Innovator Award Project Abstract
University of Pennsylvania’s News Release on Drs. Gitler and Shorter’s recent work
H. J. Kim et al., Mutations in prion-like domains in hnRNPA2B1 and hnRNPA1 cause multisystem proteinopathy and ALS. Nature 495, 467 (2013).
O. D. King, A. D. Gitler, J. Shorter, The tip of the iceberg: RNA-binding proteins with prion-like domains in neurodegenerative disease. Brain research 1462, 61 (2012).
ANN HOCHSCHILD, PH.D., HARVARD MEDICAL SCHOOL
Dr. Ann Hochschild, an NIH Director’s Pioneer Awardee, recently published a paper describing a new method developed in her laboratory using bacteria to screen bacterial, yeast, and human proteins for their ability to undergo protein misfolding resulting in amyloids. In this newly described method, Dr. Hochschild uses E. coli, which naturally export amyloid proteins, using a process called the “curli export system”, to the cell surface, as a tool to determine which proteins can form amyloids and which cannot. Using this simple method, if a protein has the potential to misfold and form an amyloid, the curli export system will help convert the protein into its misfolded, amyloid state and transport those amyloids to the cell surface. If a protein is not capable of developing an amyloid state, no amyloid will develop and no amyloid accumulation will occur at the cell surface. This method, which the researchers named curli-dependant amyloid generator, C-DAG, can be used to screen large numbers of genes for their potential to form amyloid proteins.
Dr. Hochschild’s NIH Director’s Pioneer Award Project Abstract
V. Sivanathan, A. Hochschild, Generating extracellular amyloid aggregates using E. coli cells. Genes & development 26, 2659 (2012).
S. J. Garrity, V. Sivanathan, J. Dong, S. Lindquist, A. Hochschild, Conversion of a yeast prion protein to an infectious form in bacteria. Proceedings of the National Academy of Sciences of the United States of America 107, 10596 (2010).
PEDRO FERNANDEZ-FUNEZ, PH.D., UNIVERSITY OF FLORIDA
A fruit fly, Drosophila melanogaster, has served as a model for a number of studies on protein misfolding and neurodegenerative diseases for New Innovator, Dr. Pedro Fernandez-Funez’s research. Using a specific Drosophila model created in his laboratory, Dr. Fernandez-Funez has focused his research on understanding how prions actually result in neurological disease. This research has provided valuable insights into how prions interact with other host-proteins, and the complexity of how these misfolded proteins can cause disease in the brain.
Dr. Fernandez-Funez’s NIH Director’s New Innovator Award Project Abstract
D. E. Rincon-Limas, K. Jensen, P. Fernandez-Funez, Drosophila models of proteinopathies: the little fly that could. Current pharmaceutical design 18, 1108 (2012).