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!
At last, butterflies get a bigger, better evolutionary tree
16.02.2018 | Florida Museum of Natural History
New treatment strategies for chronic kidney disease from the animal kingdom
16.02.2018 | Veterinärmedizinische Universität Wien
Breakthrough provides a new concept of the design of molecular motors, sensors and electricity generators at nanoscale
Researchers from the Institute of Organic Chemistry and Biochemistry of the CAS (IOCB Prague), Institute of Physics of the CAS (IP CAS) and Palacký University...
For photographers and scientists, lenses are lifesavers. They reflect and refract light, making possible the imaging systems that drive discovery through the microscope and preserve history through cameras.
But today's glass-based lenses are bulky and resist miniaturization. Next-generation technologies, such as ultrathin cameras or tiny microscopes, require...
Scientists from the University of Zurich have succeeded for the first time in tracking individual stem cells and their neuronal progeny over months within the intact adult brain. This study sheds light on how new neurons are produced throughout life.
The generation of new nerve cells was once thought to taper off at the end of embryonic development. However, recent research has shown that the adult brain...
Theoretical physicists propose to use negative interference to control heat flow in quantum devices. Study published in Physical Review Letters
Quantum computer parts are sensitive and need to be cooled to very low temperatures. Their tiny size makes them particularly susceptible to a temperature...
Let’s say the armrest is broken in your vintage car. As things stand, you would need a lot of luck and persistence to find the right spare part. But in the world of Industrie 4.0 and production with batch sizes of one, you can simply scan the armrest and print it out. This is made possible by the first ever 3D scanner capable of working autonomously and in real time. The autonomous scanning system will be on display at the Hannover Messe Preview on February 6 and at the Hannover Messe proper from April 23 to 27, 2018 (Hall 6, Booth A30).
Part of the charm of vintage cars is that they stopped making them long ago, so it is special when you do see one out on the roads. If something breaks or...
15.02.2018 | Event News
13.02.2018 | Event News
12.02.2018 | Event News
16.02.2018 | Information Technology
16.02.2018 | Health and Medicine
16.02.2018 | Physics and Astronomy