Research has implications for understanding brain diseases caused by clumps of misshapen molecules
It's almost axiomatic that misfolded proteins compromise how cells normally function and cause debilitating human disease, but how these proteins are detected and degraded within the body is not well understood. Neurodegenerative diseases – including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis (Lou Gehrig's disease), Huntington's disease, and spinocerebellar ataxias – exact a devastating toll on aging populations throughout the world.
This is a model for protein quality control by the PML-RNF4 system.
Credit: Lili Guo, Ph.D., Perelman School of Medicine, University of Pennsylvania; Molecular Cell
"Yet, there is virtually no cure for any of these diseases, and clinical trials have yielded mostly disappointing results, indicating that investigators are missing something fundamental about these diseases," says Xiaolu Yang, PhD, professor of Cancer Biology, Perelman School of Medicine at the University of Pennsylvania. All of these diseases are caused by the accumulation of toxic misfolded proteins in different types of neurons. "However, our knowledge about how cells normally remove these proteins, an issue fundamental to understanding what goes wrong in neurodegenerative diseases, is very limited."
Yang and first author Lili Guo, PhD, who was a doctoral student in the Yang lab, identified a protein recycling pathway in mammalian cells that removes misfolded proteins. Their findings appear online today in Molecular Cell ahead of the print edition. They also demonstrated this pathway's role in protecting against neurodegenerative diseases in an animal model. Guo is now a postdoctoral fellow in another Perelman School lab.
Proteins are the work horses of the cells. They are the most abundant macromolecules, extremely versatile in their functions and critically important for virtually all biological processes. However, proteins are also highly prone to misfolding due to genetic mutations, synthetic inaccuracies, and irreparable damages. What's more, the half-life of many proteins is relatively short, from a few minutes to a few hours, necessitating their continued synthesis. As such this puts a burden on the cell to maintain quality control in the correct folding of proteins.
"This paper resolves a long-standing question in protein quality control of how misfolded proteins are precisely selected for degradation," notes Yang. "This newly described system could be a valuable target for treating neurodegenerative diseases."
In any normally functioning cell, two systems maintain protein quality. First, chaperone proteins, like fingers that fold paper into origami shapes, guide amino acid chains in folding into their final, proper protein forms. Second, the recycling systems dispose of misfolded proteins and ultimately breaks them up into individual amino acids. This system involves the proteasome, a protein complex found throughout the cytoplasm and nucleus of cells. "But it is unclear how misshapen proteins are recognized and shuttled to the proteasome to be degraded. This study moves the field forward because we showed that the system is common for many types of misfolded proteins," notes Yang.
In addition to identifying the step-by-step molecular players of the system that eliminates misfolded proteins, they also defined the system's method of action. The mechanism of action is a relay system with two proteins.
The first protein, PML/TRIM19, recognizes features of misfolded proteins such as exposed water-phobic inner cores, which stick out and enhance the forming of toxic protein clumps. PML selectively interacts with misfolded proteins by recognizing this and other distinct features and tags the misfolded proteins with chains of a small protein called SUMO (small ubiquitin-like modifier). The SUMO-modified misfolded proteins are then recognized by the second protein, RNF4, which tags them with chains of another small protein called ubiquitin. The ubiquitin chain is a signal that can be recognized by the barrel-shaped proteasome, leading to the degradation of misfolded proteins.
The team then went on to demonstrate the physiological importance of the elimination system using a mouse model of spinocerebellar ataxia 1 (SCA1), a fatal neurological disorder that causes problems with movement and balance. Mutations in the ATXN1 gene cause SCA1, which involves repeated segments of the DNA building blocks cytosine (C), adenine (A), and guanine (G) that appear multiple times in a row in the gene, which encodes a tract of contiguous glutamine amino acids. Normally, the CAG segment is repeated 4 to 39 times within the gene. In SCA1, the CAG segment is repeated 40 to more than 80 times, which leads to an abnormally long ataxin-1 protein that folds into the wrong 3-dimensional shape and forms clumps within the nucleus. The team showed that a PML deficiency exacerbates both the behavioral and neuropathological defects caused by the expanded CAG repeat in the SCA1 mouse, indicating that PML normally operates to protect against neurodegeneration.
The PML protein is also involved in the formation of tumors, and that's how Yang, a cancer biologist, got involved in this type of research.
"The knowledge gained from this project now provides for the possibility of new therapeutic targets for neurodegenerative diseases. Perhaps new drugs could enhance the elimination system by increasing the action of PML or RNF4, or could inhibit the inhibitors of the SUMO and ubiquitin tagging process," says Yang
Other co-authors are Alex Glavis-Bloom, Michael D. Brewer, James Shorter, Aaron D. Gitler, now at Stanford Medical Center, and Benoit I. Giasson, now at the University of Florida.
The research was supported in part by grants from the National Cancer Institute (CA088868) and the National Institute of General Medical Sciences (GM060911).
Penn Medicine is one of the world's leading academic medical centers, dedicated to the related missions of medical education, biomedical research, and excellence in patient care. Penn Medicine consists of the Raymond and Ruth Perelman School of Medicine at the University of Pennsylvania (founded in 1765 as the nation's first medical school) and the University of Pennsylvania Health System, which together form a $4.3 billion enterprise.
The Perelman School of Medicine has been ranked among the top five medical schools in the United States for the past 17 years, according to U.S. News & World Report's survey of research-oriented medical schools. The School is consistently among the nation's top recipients of funding from the National Institutes of Health, with $392 million awarded in the 2013 fiscal year.
The University of Pennsylvania Health System's patient care facilities include: The Hospital of the University of Pennsylvania -- recognized as one of the nation's top "Honor Roll" hospitals by U.S. News & World Report; Penn Presbyterian Medical Center; Chester County Hospital; Penn Wissahickon Hospice; and Pennsylvania Hospital -- the nation's first hospital, founded in 1751. Additional affiliated inpatient care facilities and services throughout the Philadelphia region include Chestnut Hill Hospital and Good Shepherd Penn Partners, a partnership between Good Shepherd Rehabilitation Network and Penn Medicine.
Penn Medicine is committed to improving lives and health through a variety of community-based programs and activities. In fiscal year 2013, Penn Medicine provided $814 million to benefit our community.
Karen Kreeger | Eurek Alert!
Discovery of a new protein gives insight into a long-standing plant immunity mystery
09.12.2019 | American Phytopathological Society
How a penalty shootout is decided in the brain
09.12.2019 | Deutsches Primatenzentrum GmbH - Leibniz-Institut für Primatenforschung
Using a clever technique that causes unruly crystals of iron selenide to snap into alignment, Rice University physicists have drawn a detailed map that reveals...
University of Texas and MIT researchers create virtual UAVs that can predict vehicle health, enable autonomous decision-making
In the not too distant future, we can expect to see our skies filled with unmanned aerial vehicles (UAVs) delivering packages, maybe even people, from location...
With ultracold chemistry, researchers get a first look at exactly what happens during a chemical reaction
The coldest chemical reaction in the known universe took place in what appears to be a chaotic mess of lasers. The appearance deceives: Deep within that...
Abnormal scarring is a serious threat resulting in non-healing chronic wounds or fibrosis. Scars form when fibroblasts, a type of cell of connective tissue, reach wounded skin and deposit plugs of extracellular matrix. Until today, the question about the exact anatomical origin of these fibroblasts has not been answered. In order to find potential ways of influencing the scarring process, the team of Dr. Yuval Rinkevich, Group Leader for Regenerative Biology at the Institute of Lung Biology and Disease at Helmholtz Zentrum München, aimed to finally find an answer. As it was already known that all scars derive from a fibroblast lineage expressing the Engrailed-1 gene - a lineage not only present in skin, but also in fascia - the researchers intentionally tried to understand whether or not fascia might be the origin of fibroblasts.
Fibroblasts kit - ready to heal wounds
Research from a leading international expert on the health of the Great Lakes suggests that the growing intensity and scale of pollution from plastics poses serious risks to human health and will continue to have profound consequences on the ecosystem.
In an article published this month in the Journal of Waste Resources and Recycling, Gail Krantzberg, a professor in the Booth School of Engineering Practice...
03.12.2019 | Event News
15.11.2019 | Event News
15.11.2019 | Event News
09.12.2019 | Physics and Astronomy
09.12.2019 | Physics and Astronomy
09.12.2019 | Power and Electrical Engineering