Researchers find possible link between autism and nuclear receptor protein LXRβ

Research by University of Houston scientists discovered a possible link between nuclear receptor protein LXRβ (Liver X receptor Beta) and autism spectrum disorder. They found that nuclear receptor LXRβ deletion causes poor development of dentate gyrus, a part of brain’s hippocampus. The dentate gyrus, or DG, is responsible for emotion and memory and is known to be involved in autism spectrum disorders (ASD).

This study is led by Margaret Warner and Jan-Åke Gustafsson. The results are published in journal Proceedings of the National Academy of Sciences.”Our findings suggest early changes in dentate gyrus neurogenesis ultimately provide an aberrant template upon which to build the circuitry that is involved in normal social function,” said Warner.

Researchers conducted an animal study using LXRβ-deficient mice. Behavior analysis of these mice showed autistic-like behaviors, including social interaction deficits and repetitive behavior.”Knocking out LXRβ led to autistic behavior and reduced cognitive flexibility,” said Warner. “In this paper, we share our findings that that deletion of the LXRβ (Liver X receptor Beta) causes hypoplasia or underdevelopment in the dentate gyrus and autistic-like behaviors, including abnormal social interaction and repetitive behavior.”

Citation: Cai, Yulong, Xiaotong Tang, Xi Chen, Xin Li, Ying Wang, Xiaohang Bao, Lian Wang, Dayu Sun, Jinghui Zhao, Yan Xing, Margaret Warner, Haiwei Xu, Jan-Åke Gustafsson, and Xiaotang Fan. “Liver X receptor β regulates the development of the dentate gyrus and autistic-like behavior in the mouse.” Proceedings of the National Academy of Sciences, 2018, 201800184. doi:10.1073/pnas.1800184115.

Adapted from press release by the University of Houston.

Research shows key role of FoxO proteins in osteoarthritis development

Research from scientists at The Scripps Research Institute explains why the risk of osteoarthritis increases as we age and offers a potential avenue for developing new treatments. The study’s findings suggest that FOXO proteins are responsible for the maintenance of healthy cells in the cartilage of our joints. The results are published in journal Science Translational Medicine.

“We discovered that FoxO transcription factors control the expression of genes that are essential for maintaining joint health,” says Martin Lotz, MD, a TSRI professor and senior author of the study. “Drugs that boost the expression and activity of FoxO could be a strategy for preventing and treating osteoarthritis.”

Previous research from Lotz’ lab showed that as joints age, levels of FoxO proteins in cartilage decrease. Lotz and his colleagues had also found that people with osteoarthritis have a lower expression of the genes needed for a process called autophagy. Autophagy is a cell’s way of removing and recycling its own damaged structures to stay healthy.

For the new study, researchers used mouse models with FoxO deficiency in cartilage to see how the FoxO proteins affect maintenance of cartilage throughout adulthood. The researchers noticed a striking difference in the mice with “knockout” FoxO deficiency. Their cartilage degenerated at much younger age than in control mice. The FoxO-deficient mice also had more severe forms of post-traumatic osteoarthritis induced by meniscus damage (an injury to the knee), and these mice were more vulnerable to cartilage damage during treadmill running.

The FoxO-deficient mice had defects in autophagy and in mechanisms that protect cells from damage by molecules called oxidants. Specific to cartilage, FoxO-deficient mice did not produce enough lubricin, a lubricating protein that normally protects the cartilage from friction and wear. This lack of lubricin was associated with a loss of healthy cells in a cartilage layer of the knee joint called the superficial zone.

These problems all came down to how FoxO proteins work as transcription factors to regulate gene expression. Without FoxO proteins running the show, expression of inflammation-related genes skyrockets, causing pain, while levels of autophagy-related genes plummet, leaving cells without a way to repair themselves. “The housekeeping mechanisms, which keeps cells healthy, were not working in these knockout mice,” Lotz explains.

To determine whether targeting FoxO has therapeutic benefits, the investigators used genetic approaches to increase FoxO expression in cells of humans with osteoarthritis and found that the levels of lubricin and protective genes returned to normal. The next step in this research is to develop molecules that enhance FoxO and test them in experimental models of osteoarthritis.

Citation: Matsuzaki, Tokio, Oscar Alvarez-Garcia, Sho Mokuda, Keita Nagira, Merissa Olmer, Ramya Gamini, Kohei Miyata, Yukio Akasaki, Andrew I. Su, Hiroshi Asahara, and Martin K. Lotz. “FoxO transcription factors modulate autophagy and proteoglycan 4 in cartilage homeostasis and osteoarthritis.” Science Translational Medicine 10, no. 428 (2018). doi:10.1126/scitranslmed.aan0746.

Research funding: NIH

Adapted from press release by The Scripps Research Institute.

Large synthetic nanoparticles mimic biomolecules

Chemists at Carnegie Mellon University have demonstrated that synthetic nanoparticles can achieve the same level of structural complexity, hierarchy and accuracy as their natural counterparts biomolecules. The study, published in Science, also reveals the atomic-level mechanisms behind nanoparticle self-assembly.

The structure of the largest gold nanoparticle to-date, Au246(SR)80, was resolved using x-ray crystallography.
Credit: Rongchao Jin Carnegie Mellon University.

The findings from the lab of Chemistry Professor Rongchao Jin provide researchers with an important window into how nanoparticles form, and will help guide the construction of nanoparticles, including those that can be used in the fabrication of computer chips, creation of new materials, and development of new drugs and drug delivery devices.

“Most people think that nanoparticles are simple things, because they are so small. But when we look at nanoparticles at the atomic level, we found that they are full of wonders,” said Jin.

Nanoparticles are typically between 1 and 100 nanometers in size. Particles on the larger end of the nanoscale are harder to create precisely. Jin has been at the forefront of creating precise gold nanoparticles for a decade, first establishing the structure of an ultra-small Au25 nanocluster and then working on larger and larger ones. In 2015, his lab used X-ray crystallography to establish the structure of an Au133 nanoparticle and found that it contained complex, self-organized patterns that mirrored patterns found in nature.

In the current study, they sought to find out the mechanisms that caused these patterns to form. The researchers, led by graduate student Chenjie Zeng, established the structure of Au246, one of the largest and most complex nanoparticles created by scientists to-date and the largest gold nanoparticle to have its structure determined by X-ray crystallography. Au246 turned out to be an ideal candidate for deciphering the complex rules of self- assembly because it contains an ideal number of atoms and surface ligands and is about the same size and weight as a protein molecule.

Analysis of Au246’s structure revealed that the particles had much more in common with biomolecules than size. They found that the ligands in the nanoparticles self-assembled into rotational and parallel patterns that are strikingly similar to the patterns found in proteins’ secondary structure. This could indicate that nanoparticles of this size could easily interact with biological systems, providing new avenues for drug discovery.

The researchers also found that Au246 particles form by following two rules. First, they maximize the interactions between atoms, a mechanism that had been theorized but not yet seen. Second the nanoparticles match symmetric surface patterns, a mechanism that had not been considered previously. The matching, which is similar to puzzle pieces coming together, shows that the components of the particle can recognize each other by their patterns and spontaneously assemble into the highly ordered structure of a nanoparticle.

“Self-assembly is an important way of construction in the nanoworld. Understanding the rules of self-assembly is critical to designing and building up complex nanoparticles with a wide-range of functionalities,” said Zeng, the study’s lead author.

In future studies, Jin hopes to push the crystallization limits of nanoparticles even farther to larger and larger particles. He also plans to explore the particles’ electronic and catalytic power.

Citation: Zeng, Chenjie, Yuxiang Chen, Kristin Kirschbaum, Kelly J. Lambright, and Rongchao Jin. “Emergence of hierarchical structural complexities in nanoparticles and their assembly.” Science 354, no. 6319 (2016): 1580-1584.
DOI: 10.1126/science.aak9750
Research funding: Air Force Office of Scientific Research, Camille Dreyfus Teacher-Scholar Awards Program.
Adapted from press release by Carnegie Mellon University.

Protein Wnt5a is important in sustaining adult neuron structure in mice hippocampus

Humans and other vertebrates depend on a portion of the brain called the hippocampus for learning, memory and their sense of location. Nerve cell structures in the adult hippocampus are sustained by factors whose identities have remained largely mysterious so far.

Now, research led by a Johns Hopkins University biologist Dr. Kuruvilla sheds light on the subject, potentially pointing the way to a better understanding of how the structure of nerve cells in the adult hippocampus may deteriorate, which can lead to Alzheimer’s disease and other neurological disorders.

In a paper in the journal Proceedings of the National Academy of Sciences, Kuruvilla and eight other scientists from two research institutions report that a protein that has primarily been studied for its role in early animal development also plays a surprising role in maintaining the structure of hippocampal neurons in adult mice.

The team studied a protein called Wnt5a, which belongs to a family of proteins that have been studied primarily for their functions during embryonic development and in nurturing neurons as the young brain forms. Using mice genetically altered to remove Wnt5a from the hippocampus, the team showed that the protein’s absence did not affect hippocampus development in young mice, but instead resulted in striking degradation of specific nerve cell structures called dendrites, which resemble clusters of tree branches, in adult mice. These findings suggest that the protein plays an important role in maintaining dendrite structures as the mouse ages.

The team went further by showing that when the Wnt5a protein was reintroduced after the dendrites had started to deteriorate in aged mice, the nerve cell structures were restored – to a degree the scientists did not expect.

The team also tested the ability of mice lacking Wnt5a in the hippocampus to perform learning and memory tasks. Behavior tests were run in a Morris Water Maze designed to show how well mice use spatial cues to navigate in a water pool and how long it takes them to learn to use a hidden platform to escape. They found that mutant mice – those without Wnt5a – were poor learners and had attenuated memory; their performance in behavioral tasks became progressively worse with age.

“Together, the findings from the Morris Water Maze test support an essential role for Wnt5a in the acquisition of spatial learning and memory storage in adult animals,” the authors wrote.
In the brain, the hippocampus is the seat of short- and long-term memory and governs spatial orientation. Studies have shown that the brains of people with Alzheimer’s disease have structural alterations in dendrites, particularly in the hippocampus.

Kuruvilla said the experiments suggest avenues for further research on what may cause shrinkage of dendrites during neurological disorders. She emphasized, however, that it is premature to extrapolate these results in mice to humans.

While Kuruvilla was careful not to overstate the significance of the findings or how they may apply to cognitive disorders in humans, she said the results at least bring attention to the significance of studying molecular signals that maintain neurons in the adult brain. Compared to the wealth of information on signals that help in formation of neuronal connections in the developing brain, we know far less of how brain structure is sustained in adult life, she said.

Citation: Chih-Ming Chena,  Lauren L. Oreficeb,  Shu-Ling Chiuc,  Tara A. LeGatesa,  Samer Hattara,  Richard L. Huganirc,  Haiqing Zhaoa,  Baoji Xub, and  Rejji Kuruvillaa. “Wnt5a is essential for hippocampal dendritic maintenance and spatial learning and memory in adult mice.” PNAS 2017.
DOI: 10.1073/pnas.1615792114
Research funding: National Institutes of Health.
Adapted from press release by Johns Hopkins University.

Research in mice shows molecular mechanism underlying Oxycodone addiction

RGS9-2, a key signaling protein in the brain known to play a critical role in the development of addiction-related behaviors, acts as a positive modulator of oxycodone reward in both pain-free and chronic pain states, according to a study conducted at the Icahn School of Medicine at Mount Sinai and published in the journal Neuropsychopharmacology. The mechanisms of oxycodone action uncovered through this study will help scientists and physicians develop strategies and tools to dissociate the analgesic (pain relief) actions of opioids from the addiction-related effects.

Pixabay images

Using mouse models of acute and chronic pain, Mount Sinai researchers found that RGS9-2, the intracellular protein that controls the function of opioid receptors in the brain reward center, promotes addiction to oxycodone in pain-free, acute, and chronic pain states. Mice that lacked the gene responsible for encoding RGS9-2 (RGS9KO mice) showed less propensity to develop addiction-related behaviors. Furthermore, the loss of RGS9-2 function does not affect the acute analgesic effects of oxycodone. The research team also found that RSG9-2 plays a protective role towards the development of oxycodone tolerance, as RGS9KO mice became tolerant to the analgesic effects of the drug earlier than those that had the gene. Researchers found that the same mechanisms control sensitivity to oxycodone addiction in pain-free as well as chronic pain states.

Oxycodone is a painkiller that is widely prescribed for acute and chronic pain conditions and is also among the most abused opioids. Oxycodone acts on the same brain receptors as morphine and heroin, the mu opioid receptors, which are present in many areas of the brain that mediate pain relief but are also expressed in the brain network associated with addiction. While there has been an extensive investigation into the mechanisms underlying the analgesia, dependence, and addiction potential of morphine, the mechanism by which oxycodone exerts its actions remained unknown.

“Although oxycodone produces similar analgesic and behavioral effects to those observed with morphine, our study demonstrates that the intracellular actions of morphine and oxycodone are distinct,” says Venetia Zachariou, Ph.D., Associate Professor in the Fishberg Department of Neuroscience and The Friedman Brain Institute, Icahn School of Medicine at Mount Sinai. “Our work reveals that intracellular factors that prevent the actions of morphine may actually promote the actions of oxycodone. This information is particularly important for pain management strategies, as a common course is to have patients oscillate between oxycodone and morphine to achieve pain relief.”

This study provides new information on pathways involved in behavioral responses to oxycodone in pain-free and neuropathic pain states, which will help researchers and clinicians to determine the risks and benefits of oxycodone prescription for the treatment of pain. This knowledge may lead to the development of more efficacious and less addictive compounds for pain management.

Citation: Sevasti Gaspari, Valeria Cogliani, Lefteris Manouras, Ethan M Anderson, Vasiliki Mitsi, Kleopatra Avrampou, Fiona B Carr and Venetia Zachariou. “RGS9-2 Modulates Responses to Oxycodone in Pain-Free and Chronic Pain States.” Neuropsychopharmacology 2017.
DOI: 10.1038/npp.2017.4
Research funding: National Institute of Neurological Disorders and Stroke
Adapted from press release by The Mount Sinai Hospital.

Role of retroviruses in evolution of human brain

Over millions of years, retroviruses have been incorporated into our human DNA, where they today make up almost 10 percent of the total genome. A research group at Lund University in Sweden has now discovered a mechanism through which these retroviruses may have an impact on gene expression. This means that they may have played a significant role in the development of the human brain as well as in various neurological diseases.

Retroviruses are a special group of viruses including some which are dangerous, such as HIV, while others are believed to be harmless. The viruses studied by Johan Jakobsson and his colleagues in Lund are called endogenous retroviruses (ERV) as they have existed in the human genome for millions of years. They can be found in a part of DNA that was previously considered unimportant, so-called junk-DNA a notion that researchers have now started to reconsider.

The genes that control the production of various proteins in the body represent a smaller proportion of our DNA than endogenous retroviruses. They account for approximately 2 per cent, while retroviruses account for 8-10 per cent of the total genome. If it turns out that they are able to influence the production of proteins, this will provide us with a huge new source of information about the human brain”, says Johan Jakobsson.

And this is precisely what the researchers discovered. They have determined that several thousands of the retroviruses that have established themselves in our genome may serve as “docking platforms” for a protein called TRIM28. This protein has the ability to “switch off” not only viruses but also the standard genes adjacent to them in the DNA helix, allowing the presence of ERV to affect gene expression.

This switching-off mechanism may behave differently in different people since retroviruses are a type of genetic material that may end up in different places in the genome. This makes it a possible tool for evolution, and even a possible underlying cause of neurological diseases. In fact, there are studies that indicate a deviating regulation of ERV in several neurological diseases such as ALS, schizophrenia, and bipolar disorder.

Two years ago, Johan Jakobsson’s team showed that ERV had a regulatory role in neurons specifically. However, this study was conducted on mice, whereas the new study published in the journal Cell Reports was made using human cells.

The differences between mice and humans are particularly important in this context. Many of the retroviruses that have been built into the human DNA do not exist in species other than humans and our closest relatives gorillas and chimpanzees. They seem to have incorporated themselves into the genome some 35-45 million years ago when the evolutionary lineage of primates was divided between the Old and New World.

“Much of what we know about the overall development of the brain comes from the fruit fly, zebrafish, and mouse. However, if endogenous retroviruses affect brain function, and we have our own set of these ERV, the mechanisms they affect may have contributed to the development of the human brain”, says Johan Jakobsson.

Citation: Brattås, Per Ludvik, Marie E. Jönsson, Liana Fasching, Jenny Nelander Wahlestedt, Mansoureh Shahsavani, Ronny Falk, Anna Falk, Patric Jern, Malin Parmar, Johan Jakobsson. “TRIM28 Controls a Gene Regulatory Network Based on Endogenous Retroviruses in Human Neural Progenitor Cells.” Cell reports Volume 18, Issue 1, p1–11.
DOI: 10.1016/j.celrep.2016.12.010
Adapted from press release by Lund University.

Research unveils structure of crucial bacterial cell wall protein

Duke University researchers have provided the first close-up glimpse of a protein, called MurJ, which is crucial for building the bacterial cell wall and protecting it from outside attack. The research is published in Nature Structural and Molecular Biology.

Researchers at Duke University solved the structure of an enzyme that is crucial for helping bacteria build their cell walls. The molecule, called MurJ (shown in green), must flip cell wall precursors (purple) across the bacteria’s cell membrane before these molecules can be linked together to form the cell wall. This new structure could be important to help develop new broad-spectrum antibiotics. Credit: Alvin Kuk, Duke University

“Until now, MurJ’s mechanisms have been somewhat of a ‘black box’ in the bacterial cell wall synthesis because of technical difficulties studying the protein,” said senior author Seok-Yong Lee, Ph.D., associate professor of biochemistry at Duke University School of Medicine. “Our study could provide insight into the development of broad spectrum antibiotics, because nearly every type of bacteria needs this protein’s action.”

A bacterium’s cell wall is composed of a rigid mesh-like material called peptidoglycan. Molecules to make peptidoglycan are manufactured inside the cell and then need to be transported across the cell membrane to build the outer wall.

In 2014, another group of scientists had discovered that MurJ is the transporter protein located in the cell membrane that is responsible for flipping these wall building blocks across the membrane. Without MurJ, peptidoglycan precursors build up inside the cell and the bacterium falls apart. Many groups have attempted to solve MurJ’s structure without success, partly because membrane proteins are notoriously difficult to work with.

In this study, Lee’s team was able to crystallize MurJ and determine its molecular structure to 2-angstrom resolution by an established method called X-ray crystallography, which is difficult to achieve in a membrane protein. The structure, combined with follow-up experiments in which the scientists mutated specific residues of MurJ, allowed them to propose a model for how it flips peptidoglycan precursors across the membrane.

After determining the first structure of MurJ, Lee’s team is now working to capture MurJ in action, possibly by crystallizing the protein while it is bound to a peptidoglycan precursor. “Getting the structure of MurJ linked to its substrate will be key. It will really help us understand how this transporter works and how to develop an inhibitor targeting this transporter,” Lee said.

Lee’s group is continuing structure and function studies of other key players in bacterial cell wall biosynthesis as well. Last year, they published the structure of another important enzyme, MraY, bound to the antibacterial muraymycin.

Citation: Kuk, Alvin CY, Ellene H. Mashalidis, and Seok-Yong Lee. “Crystal structure of the MOP flippase MurJ in an inward-facing conformation.” Nature Structural & Molecular Biology (2016).
DOI: 10.1038/nsmb.3346
Research funding: Duke University
Adapted from press release by the Duke University.

Computer models to analyze Huntington disease pathology

Rice University scientists have uncovered new details about how a repeating nucleotide sequence in the gene for a mutant protein may trigger Huntington’s and other neurological diseases. Researchers used computer models to analyze proteins suspected of misfolding and forming plaques in the brains of patients with neurological diseases. Their simulations confirmed experimental results by other labs that showed the length of repeating polyglutamine sequences contained in proteins is critical to the onset of disease. The study led by Rice bioscientist Peter Wolynes appears in the Journal of the American Chemical Society.

Glutamine is the amino acid coded for by the genomic trinucleotide CAG. Repeating glutamines, called polyglutamines, are normal in huntingtin proteins, but when the DNA is copied incorrectly, the repeating sequence of glutamines can become too long. The result can be diseases like Huntington’s or spinocerebellar ataxia.

Simulations at Rice show how a repeating sequence in a mutant
 protein may trigger Huntington’s and other neurological diseases.
Credit:Mingchen Chen/Rice University

The number of repeats of glutamine can grow as the genetic code information is passed down through generations. That means a healthy parent whose huntingtin gene encodes proteins with 35 repeats may produce a child with 36 repeats. A person having the longer repeat is likely to develop Huntington’s disease.

Aggregation in Huntington’s typically begins only when polyglutamine chains reach a critical length of 36 repeats. Studies have demonstrated that longer repeat chains can make the disease more severe and its onset earlier.

The paper builds upon techniques used in an earlier study of amyloid beta proteins. That study was the lab’s first attempt to model the energy landscape of amyloid aggregation, which has been implicated in Alzheimer’s disease.  This time, Wolynes and his team were interested in knowing how the varying length of repeats, as few as 20 and as many as 50 influenced how aggregates form.

The Rice team found that at intermediate lengths between 20 and 30 repeats, polyglutamine sequences can choose between straight or hairpin configurations. While longer and shorter sequences form aligned fiber bundles, simulations showed intermediate sequences are more likely to form disordered, branched structures.

Mutations that would encourage polyglutamine sequences to remain unfolded would raise the energy barrier to aggregation, they found. “What’s ironic is that while Huntington’s has been classified as a misfolding disease, it seems to happen because the protein, in the bad case of longer repeats, carries out an extra folding process that it wasn’t supposed to be doing,” Wolynes said.

The team’s ongoing study is now looking at how the complete huntingtin protein, which contains parts in addition to the polyglutamine repeats, aggregates.

Citation: Chen, Mingchen, MinYeh Tsai, Weihua Zheng, and Peter G. Wolynes. “The Aggregation Free Energy Landscapes of Polyglutamine Repeats.” Journal of the American Chemical Society (2016).
DOI: 10.1021/jacs.6b08665
Research funding: NIH/National Institute of General Medical Sciences, Ministry of Science and Technology of Taiwan
Adapted from press release by Rice University.

Role of nerve growth factor in glucose metabolism

Research led by a Johns Hopkins University biologist demonstrates the workings of a biochemical pathway that helps control glucose in the bloodstream, a development that could potentially lead to treatments for diabetes.

In a paper published in the current issue of Developmental Cell, Jessica Houtz, a graduate student working with Rejji Kuruvilla in the Department of Biology at Johns Hopkins, shows that a protein that regulates the development of nerve cells also plays a role in prompting cells in the pancreas to release insulin, a hormone that helps to maintain a normal level of blood sugar. The research on insulin represents a detour for Kuruvilla, whose work has focused on development of the peripheral nervous system. She has studied a group of proteins called neurotrophins, and in particular nerve growth factor [NGF]. These proteins nurture the growth of neurons, the cells of the nervous system.

It has been known for some time that neurons and the pancreatic beta cells, or β-cells, that reside in clusters called islets of Langerhans and produce insulin, have many similarities in molecular makeup and signaling receptors. Receptors are proteins on cell surfaces that respond to particular chemicals and have critical roles in biochemical pathways. Both neurons and pancreatic β-cells have the receptors for neurotrophins.

It turns out that particular nerve growth factor [NGF] performs a function in the mature pancreas that has nothing to do with supporting neurons. Specifically, the research team traced a chain of biochemical signals showing that elevated blood glucose causes particular nerve growth factor  to be released from blood vessels in the pancreas, and that the particular nerve growth factor signal then prompts pancreatic β-cells to relax their rigid cytoskeletal structure, releasing insulin granules into the blood stream. Although β-cells also makeparticular nerve growth factor, Kuruvilla and her team found that it was the particular nerve growth factor released from the blood vessels that is needed for insulin secretion.

Using genetic manipulation in mice and drugs to block particular nerve growth factor [NGF] signaling in β-cells, they were able to disrupt distinct elements of this signaling sequence, to show that this classical neuronal pathway is necessary to enhance insulin secretion and glucose tolerance in mice. Importantly, Kuruvilla and colleagues found that particular nerve growth factor’s ability to enhance insulin secretion in response to high glucose also occurs in human β-cells.

It is not yet clear how this system is affected in people with diabetes. “We are very interested in knowing whether aspects of this pathway are disrupted in pre-diabetic individuals,” Kuruvilla said. It would be of interest to determine if particular nerve growth factor [NGF] or small molecules that bind and activate particular nerve growth factor receptors in the pancreas could be of potential use in the treatment of type-2 diabetes. These are questions to be pursued in further research.

Citation: Houtz, Jessica, Philip Borden, Alexis Ceasrine, Liliana Minichiello, and Rejji Kuruvilla. “Neurotrophin Signaling Is Required for Glucose-Induced Insulin Secretion.” Developmental Cell 39, no. 3 (2016): 329-345.
DOI:http://dx.doi.org/10.1016/j.devcel.2016.10.003 
Research funding: NIH
Adapted from press release by John Hopkins University

Researchers discover how selenium is incorporated into proteins

Humans need eight essential trace elements for good health, and one of them is selenium – a powerful antioxidant that is important for thyroid and brain function as well as metabolism. But trace elements can’t be used by the body until they are integrated into a protein molecule. Selenium is unique because it is folded into its protein while the protein molecule is still being made. All other trace elements are added to their respective protein molecules after the cell has finished synthesizing the protein. Researchers at the University of Illinois at Chicago have discovered  how selenium is incorporated into selenoproteins. The finding is published in the journal Nature Communications.

Proteins are made by linking amino acids together, one at a time, in a chain. Cellular structures called ribosomes serve as docking stations, where all the components involved in protein production come together — messenger RNA, which serves as the protein’s blueprint; the amino acid building blocks, each attached to its own specific transfer RNA; and various helper molecules. Elongation factors are an important type of helper protein that guide the amino acids to the ribosome during protein synthesis. In humans, elongation factor eEF1A helps string together amino acids at the ribosome – that is, all amino acids except selenocysteine, the amino acid that holds selenium. In humans, selenocysteine is incorporated into proteins with help of a unique elongation factor called eEFSec that works very differently from eEF1A.

“We’ve known that selenium is special when it comes to protein synthesis, because there is a whole other set of rules and tools in use,” says Miljan Simonovic, associate professor of biochemistry and molecular genetics in the UIC College of Medicine and corresponding author on the paper. “Not only does it have its own elongation factor, but selenocysteine is also very unusual because it is represented in the genetic code by the same three-letter key, or codon, that signals for protein synthesis to stop.”  Normally, as the ribosome reads the messenger RNA — or mRNA — and reaches this stop codon, it detaches from the mRNA because its work is done, although the full-length protein may still be modified through other processes.  But sometimes the ribosome runs the stop sign and adds selenocysteine instead — and continues to elongate the protein until it reaches another stop sign.

“When the stop codon means ‘bring in a selenocysteine,’ additional protein factors together with structural features in the mRNA around that stop codon, such as loops, indicate to the ribosome not to stop selenoprotein production,” said Malgorzata Dobosz-Bartoszek, postdoctoral research associate in biological sciences, who is lead author on the paper. “The selenocysteine elongation factor, eEFSec, plays a key role in helping to recognize the stop codon as actually coding for selenocysteine.”  Simonovic said the eEFSec elongation factor also stands apart in how it changes shape when it delivers selenocysteine to the ribosome. The researchers showed that eEFSec bends about 20 degrees when delivering selenocysteine, while eEF1A bends “much more dramatically — more like 90 degrees” when it drops off the other amino acids. 

Simonovic thinks that the reason selenocysteine is handled so differently during protein synthesis traces back to the Great Oxygenation Event. This was the period about 2.3 billion years ago when free oxygen in Earth’s atmosphere suddenly spiked, due to the evolutionary emergence of plants and photosynthesis as a way to derive energy from the sun. Organisms needed to evolve ways to prevent cellular damage caused by oxidation, and selenium, a powerful antioxidant, would have been available. But already-existing processes for incorporating trace elements into proteins may not have worked for selenium, which is extremely reactive.  “We know that eEFSec has a unique domain that helps it safely interact with selenocysteine,” Simonovic said.

Publication: Crystal structures of the human elongation factor eEFSec suggest a non-canonical mechanism for selenocysteine incorporation
doi:10.1038/ncomms12941
Adapted from press release by University of Illinois at Chicago