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glucocorticoid receptor

Promiscuous protein droplets regulate immune gene activity

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.

These are fluorescent droplets of the glucocorticoid receptor (GR) in red, with a coregulator protein in green. When DNA is added, the co-regulator forms its own droplets on the surface of GR droplets. Image courtesy of Filipp Frank

“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.”

The results are published in Proceedings of the National Academy of Sciences. Ortlund is a co-author of the paper, along with postdoctoral fellow Xu Liu, PhD.

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.

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Posted on by Quinn Eastman in Cancer, Immunology Leave a comment

Tweaks to corticosteroids may reduce side effects

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.

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Ancient protein flexibility may drive ‘new’ functions

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 GR’s ability to adopt different shapes.

“What 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,” 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 cells’ 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

Posted on by Quinn Eastman in Heart, Immunology Leave a comment

Resurrecting an ancient receptor to understand a modern drug

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 “resurrecting” ancient proteins to get around difficulties in probing their structures.

Steroid receptor evolution

Ortlund’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.

“We 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,” 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.

“The ancestral receptors are more tolerant to mutation, and they are more promiscuous with respect to activation,” Ortlund says. “That 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.”

The blog Panda’s Thumb has an interesting discussion of this area of research, in relation to the larger question of how proteins evolve.

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Evolution doesn’t run backwards: Insights from protein structure

“The past is difficult to recover because it was built on the foundation of its own history, one irrevocably different from that of the present and its many possible futures.”

Whoa. This quote comes from a recent Nature paper. How did studying the protein that helps cells respond to the stress hormone cortisol inspire such philosophical language?

Biochemist Eric Ortlund at Emory and collaborator Joe Thornton at the University of Oregon specialize in “resurrecting”and characterizing ancient proteins. They do this by deducing how similar proteins from different organisms evolved from a common root, mutation by mutation. Sort of like a word ladder puzzle.

Ortlund and Thornton have been studying the glucocorticoid receptor, a protein that binds the hormone cortisol and turns on genes in response to stress. The glucocorticoid receptor is related to the mineralocorticoid receptor, which binds hormones such as aldosterone, a regulator of blood pressure and kidney function.

If these receptors have a common ancestor, you can model each step in the transformation that led from the ancestor to each descendant. But Ortlund says that protein evolution isn’t like a word ladder puzzle, which can be turned upside-down: “You can’t rewind the tape of life and have it take the same path.”

The reason: Mutations arise amidst a background of selective pressure, and mutations in one part of a protein set the stage for whether other ones will be viable. The researchers describe this as an “epistatic rachet”.

Mutations that occurred during the transformation between the ancestral protein (green) and its descendant (orange) would clash if put back to their original position.

Mutations that occurred during the transformation between the ancestral protein (green) and its descendant (orange) would clash if put back to their original position.

This work highlights the increasing number of structural biologists like Ortlund, Christine Dunham, Graeme Conn and Xiaodong Cheng at Emory. Structural biologists use techniques such as X-ray crystallography to figure out how the parts of biology’s machines fit together. Recently Emory has been investing in the specialized equipment necessary to conduct X-ray crystallography.

As part of his future plans, Ortlund says he wants to go even further back in evolution, to examine the paths surrounding the estrogen receptor, which is also related to the glucocorticoid receptor.

Besides giving insight into the mechanisms of evolution, Ortlund says his research could also help identify drugs that activate members of this family of receptors more selectively. This could address side effects of drugs now used to treat cancer such as tamoxifen, for example, as well as others that treat high blood pressure and inflammation.

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