Biochemists at Emory are achieving insights into how an important regulator of the immune system switches its function, based on its orientation and local environment. New research demonstrates that the glucocorticoid receptor (or GR) forms droplets or “condensates” that change form, depending on its available partners.
The inside of a cell is like a crowded nightclub or party, with enzymes and other proteins searching out prospective partners. The GR is particularly well-connected and promiscuous, and has the potential to interact with many other proteins. It is a type of protein known as a transcription factor, which turns some genes on and others off, depending on how it is binding DNA.
“It is now thought that most transcription factors form or are recruited into condensates, and that condensation modulates their function,” says Filipp Frank, PhD, first author of the paper and a postdoctoral instructor in Eric Ortlund’s lab in the Department of Biochemistry. “What’s new is that we identified a DNA-dependent change in GR condensates, which has not been described for other transcription factors.”
Understanding how the GR works could help researchers find anti-inflammatory drugs with reduced side effects. The GR is the target for corticosteroid drugs such as dexamethasone, which is currently used to treat COVID-19 as well as allergies, asthma and autoimmune diseases.
Corticosteroids’ harmful side effects are thought to come from turning on genes involved in metabolism and bone growth, while their desired anti-inflammatory effects result from turning other inflammatory and immune system genes off. Researchers want to find alternatives that could separate those two functions.
Steroid anti-inflammatory drugs such as dexamethasone and prednisone are widely used to treat conditions such as allergies, asthma, autoimmune diseases, cancer – and now, COVID-19. Yet they can have harmful side effects on the skin, bones and metabolism.
The side effects are thought to come from a molecular mechanism that is separate from the anti-inflammatory one, and scientists have envisioned that it may be possible to divide the two. A new paper in PNAS from Emory biochemist Eric Ortlund’s lab sketches out how one potential alternative may work.
Synthetic corticosteroids mimic the action of the stress hormone cortisol; both bind the glucocorticoid receptor (GR) protein. Ortlund’s group obtained structural information on how vamorolone, an experimental drug, sticks to the part of GR that binds hormones.
The American company ReveraGen and Swiss partner Santhera are developing vamorolone for Duchenne muscular dystrophy, but it is possible to envision several other conditions such as ulcerative colitis for which vamorolone or a similar drug could be helpful. Vamorolone is NOT approved by the FDA for Duchenne muscular dystrophy or any other indication.
As far as its interaction with GR, what sets vamorolone apart from conventional corticosteroids is quite subtle: a missing hydrogen bond. This means that GR doesn’t interact as well with various partner proteins, which are needed to turn on genes involved in processes such as metabolism and bone growth. However, the anti-inflammatory effects result mainly from turning inflammatory and immune system genes off, and those interactions are maintained. More on that distinction here and here.
When thinking about the evolution of female and male, consider that the first steroid receptor proteins, which emerged about 550 million years ago, were responsive to estrogen. The ancestor of other steroid hormone receptors, responsive to hormones such as testosterone, progesterone and cortisol, emerged many millions of years later.
Biochemist Eric Ortlund and colleagues have a new paper in Structurethat reconstructs how interactions of steroid receptor proteins evolved over time. This is a complex area to model, since the receptors change shape when they bind their respective hormones, allowing them to bring in other proteins and activate genes.
First author C. Denise Okafor, a FIRST postdoctoral fellow at Emory, will be starting a position as assistant professor at Penn State next month.
A mechanism by which stress hormones inhibit the immune system, which appeared to be relatively new in evolution, may actually be hundreds of millions of years old.
A protein called the glucocorticoid receptor or GR, which responds to the stress hormone cortisol, can take on two different forms to bind DNA: one for activating gene activity, and one for repressing it. In a paper published Dec. 28 in PNAS, scientists show how evolutionary fine-tuning has obscured the origin of GRâ€™s ability to adopt different shapes.
â€œWhat this highlights is how proteins that end up evolving new functions had those capacities, because of their flexibility, at the beginning of their evolutionary history,â€ says lead author Eric Ortlund, PhD, associate professor of biochemistry at Emory University School of Medicine.
GR is part of a family of steroid receptor proteins that control cellsâ€™ responses to hormones such as estrogen, testosterone and aldosterone. Our genomes contain separate genes encoding each one. Scientists think that this family evolved by gene duplication, branch by branch, from a single ancestor present in primitive vertebrates. Read more
It arises from what scientists previously described as “junk DNA” or “the dark matter of the genome,” but this gene is definitely not junk. The gene Gas5 acts as a brake on steroid hormone receptors, making it a key player in diseases such as hormone-sensitive prostate and breast cancer.
Emory researchers have obtained a detailed picture of how the Gas5 RNA interacts with steroid hormone receptors. Their findings show how the Gas5 RNA takes the place of DNA, and give hints as to how it evolved.
Scientists used to think that much of the genome was “fly-over country”: not encoding any protein and not even accessed much by the cell’s gene-reading machinery. Recent studies have revealed that a large part of the genome is copied into lincRNAs (long intergenic noncoding RNAs), of which Gas5 is an example. Read more
To make progress in structural biology, look millions of years into the past. Emory biochemist Eric Ortlund and his colleagues have been taking the approach of â€œresurrectingâ€ ancient proteins to get around difficulties in probing their structures.
Steroid receptor evolution
Ortlundâ€™s laboratory recently published a paper in Journal of Biological Chemistry describing the structure of a protein that is supposed to have existed 450 million years ago, in a complex with an anti-inflammatory drug widely used today. MSP graduate student Jeffrey Kohn is the first author.
Mometasone furoate is the active ingredient of drugs used to treat asthma, allergies and skin irritation. It is part of a class of drugs known as glucocorticoids, which can have a host of side effects such as reduced bone density and elevated blood sugar or blood pressure with long-term use.
One reason for these side effects is because the steroid receptor proteins that allow cells to detect and respond to hormones such as estrogen, testosterone, aldosterone and cortisol are all related. Mometasone is a good example of how glucocorticoids cross-react, Ortlund says. That made it an ideal test of the technique of mixing ancient receptors with modern drugs.
â€œWe used this structure to determine why mometasone cross reacts with the progesterone receptor, which regulates fertility, and why it inhibits the mineralocorticoid receptor, which regulates blood pressure,â€ he says.
Mometasone furoate in complex with the ancient receptor
Scientists have examined the sequences of the genes that encode these proteins at several points on the evolutionary tree, and used the information to reconstruct what the ancestral receptor looked like. This helps solve some problems that biochemists studying these proteins have had to deal with. One of these is: changing one amino acid in the protein sometimes means that the whole protein malfunctions.
â€œThe ancestral receptors are more tolerant to mutation, and they are more promiscuous with respect to activation,â€ Ortlund says. â€œThat is, they tend to respond to a wider array of endogenous steroid hormones, which makes sense in an evolutionary context. This enhanced activation profile and tolerance to mutation is what we feel makes them ideally suited to structure-function studies.â€