Scientists at Scripps Research, Florida, have determined the near-atomic structure of an unusual brain cell receptor called GPR158, which has been linked to depression and anxiety.
The structural study reveals both the receptor and its regulatory complex and promotes understanding of basic cell receptor biology. It also opens the door to work on potential therapeutics to block GPR158 as a strategy for treating depression, anxiety, and possibly other mood disorders.
In the study, published Nov. 18 in the journal Science, researchers used ultra-cold single particle electron microscopy, or cryo-EM, to visualize the atomic structure of GPR158, both alone and with a resolution of about a third of a billionth of a meter bound a group of proteins that mediate its activity.
We’ve been studying this receptor for more than 10 years and have done a lot of biology with it, so it’s really nice to see for the first time how it’s organized. “
Kirill Martemyanov, PhD, lead author, professor, and chair of the Department of Neuroscience at Scripps Research
Clinical depression, also known as major depressive disorder, is estimated to affect approximately 20 million people in the United States each year. Current treatments work at other known receptors, including monoamine, but do not always work well for all people and alternative options are needed.
Martemyanov and his team found in a 2018 study that GPR158 was present in unusually high levels in the prefrontal cortex of people diagnosed with major depressive disorder at the time of death. They also found that exposure of mice to chronic stress increased levels of this receptor in the mouse’s prefrontal cortex, leading to depression-like behavior – while eliminating GPR158 activity in chronically stressed mice made them resistant to depression and the effects of stress made. In addition, the activity of the GPR158 receptor has also been linked to prostate cancer.
Historically, GPR158 has not been easy to study. It is known as the “orphan receptor” because scientists have not yet identified the molecule that is responsible for switching on its signaling function, much like flipping a switch. The receptor is also considered unusual in that, unlike most receptors in its family, it exists in the brain in close association with a protein complex called the RGS signaling complex. RGS is the abbreviation for “regulator of G-protein signaling” and acts as a powerful brake on cellular signaling. However, it was unclear why GPR158 intervened.
In the new study, solving the structure of the receptor provided a lot of insight into how GPR158 works. First, scientists found that it binds the RGS complex in the same way that many receptors normally attack their conventional transducers, leading to the idea that it uses RGS proteins as a means of transmitting its signal. Second, the structure indicated that the receptor existed as two linked copies of the GPR158 proteins that are stabilized by phospholipids.
“These are fat-related molecules that effectively glue the two halves of the receptor together,” explains Martemyanov.
Eventually, on the other side of the receptor facing the cell, an unusual module called the cache domain was discovered. The authors believe that the cache domain acts as a trap for the molecules that activate GPR158. Cache domains have never been observed before in these types of receptors, demonstrating the unique biology of this orphaned receptor.
First author Dipak Patil, PhD, a research fellow at Martemyanov’s laboratory, says that solving the structure brings a lot of new knowledge.
“I am excited to see the structure of this unique GPCR. It is the first of its kind, shows a lot of new features and offers a path for drug development, ”says Patil.
The challenge now is to use the information gleaned from the structure to design small-molecule therapeutics to fight depression, adds Martemyanov.
He is now investigating several possible approaches, including breaking the two-part arrangement, disrupting the RGS complex, or targeting the cache domain with small, drug-like molecular binders. Regardless of the path taken, the availability of structural information should greatly facilitate efforts to develop drugs to treat depression, Martemyanov says.
This study was made possible by the latest technological advances in microscopy, including freezing proteins at ultra-cold temperatures and examining their organization through the lens of powerful microscopes, a technique called cryogenic electron microscopy, or cryo-EM.
“The microscope uses an electron beam instead of light to image protein arrays. The shorter wavelength of electrons compared to light enabled us to visualize our sample with almost atomic resolution, ”says structural biologist Professor Tina Izard, PhD.
Patrick Griffin, PhD, Scripps Research, Florida Scientific Director, co-authored the study, which applied structural proteomic platform technology.
“Cryo-EM’s promise to make major breakthroughs in the dissolution of structures of biomolecules is enormous. Our institute is determined to expand cryo-EM microscopy, which is made possible by the recent acquisition and installation of a new microscope on campus. “
The study was a collaboration between researchers from Columbia University and Appu Singh, PhD, a structural biologist at the Indian Institute of Technology in Kanpur.
Scripps Research Institute
Patil, DN, et al. (2021) Cryo-EM structure of the human GPR158 receptor coupled to the RGS7-Gβ5 signal complex. Science. doi.org/10.1126/science.abl4732.