Texas A&M University scientists are providing new insight into the molecular mechanisms behind a key cellular signaling pathway involved in antiviral immune responses they believe can be exploited in the fight against diseases from coronaviruses to cancer.
In research published online earlier this month as an accelerated article in the journal Nature, Texas A&M chemist and 2018 Presidential Impact Fellow Wenshe Ray Liu and Texas A&M biochemist Pingwei Li teamed up to take a closer look at enzyme and protein-related activity within the cGAS-STING signaling pathway — specifically, the enzyme cGAS’ proclivity to bind with nucleosomes within healthy host cells to avoid autoimmune responses. Their goal was as much to understand how the enzyme protects as to explore how it prevents other health-critical processes, from DNA damage repair to cell death.
Like bacteria, Liu notes that viruses have a silent partner in their invasive missions: nucleic acid. Once released inside and detected by a host’s otherwise healthy cells, that nucleic acid triggers a potent protective immune response, providing telltale clues at the scene of every infectious crime.
Once the host’s immune system kicks into action, an enzyme known as cyclic GMP-AMP synthase (cGAS) catalyzes the synthesis of cyclic GMP-AMP (cGAMP), which then acts as a secondary “messenger” molecule by binding to the adaptor protein STING. These messenger molecules direct the activation of other protein complexes that stimulate the induction of cytokines, such as interferons, which not only kick-start the inflammation that gives immune cells quicker access to the infection site but also regulate innate antiviral immunity.
Beyond the key role it plays in antiviral immune response, Liu says the cGAS-STING signaling pathway also has been shown to be involved in cancers, autoimmune and inflammatory diseases such as systemic lupus erythematosus, and Aicardi–Goutières syndrome.
“cGAS senses DNA in a sequence-independent manner, meaning it binds to any DNA, regardless of its sequence,” Liu said. “However, cGAS does not react to host DNA, or DNA found within its own cellular genome under normal conditions. Considering there are billions of base pairs of DNA in the mammalian genome, this fact raises the question about how cGAS distinguishes between viral and host DNA.”
Li says one prevalent explanation is that cGAS is sequestered in another part of the cell known as the cytosol. Because host DNA is found exclusively in a cell’s nucleus and not the cytosol, this explanation hinges on the fact that cGAS cannot access host DNA, which is surrounded by the nuclear membrane. Liu notes this concept has been challenged by several recent studies demonstrating that cGAS also is located in the nucleus, where its activity is suppressed. In addition, it is tightly tethered to chromatin, a tightly packed form of DNA within the nucleus.
As a first step in unraveling the mysterious molecular basis of cGAS nuclear tethering, Liu and Li and their respective research groups hypothesized that the nucleosome — a DNA-protein complex that makes up the basic structural unit of chromatin — may be essential for cGAS nuclear tethering. Liu describes a nucleosome as a spool for genomic DNA, which is created when host DNA literally wraps around the histone protein core to organize and tightly pack genomic DNA.
To test their hypothesis, the team conducted binding studies with nucleosomes and the cGAS enzyme, producing results that showed cGAS binds to nucleosomes with a 100-fold higher affinity than naked DNA. Furthermore, they found that once cGAS is tightly bound to the nucleosome, it cannot catalyze cGAMP synthesis, effectively rendering the enzyme useless and thereby explaining cGAS’ inactivation by the host chromatin.
In the course of their study, the Liu-Li-led team also used cryogenic electron microscopy (cryo-EM) — a first for the Liu Laboratory— to determine the structure of cGAS-nucleosome complex. Their results revealed the DNA binding site of cGAS and the acidic patch of the nucleosome as critical surfaces involved in cGAS and nucleosome binding.
“Our structural and functional studies revealed the molecular basis for nuclear tethering and the resulting inactivation of cGAS to host DNA,” Li said. “Based on these results, we propose that tight nucleosome binding keeps cGAS in an inactive form to avoid autoimmune responses to genomic DNA.”
“Because cGAS binds nucleosomes much more tightly than free DNA, which includes non-nucleosome-bound host DNA, cGAS cannot be activated by free DNA when it is associated with the nucleosome complex,” Liu added. “This interaction likely prohibits similar interactions with other crucial nucleosome-binding proteins involved in DNA damage repair, epigenetic regulation, cancer and cell death. The cGAS-STING pathway’s precise role in those processes considering nucleosome-mediated cGAS inactivation needs to be determined through future studies.”
In addition to Liu and Li, the Department of Biochemistry and Biophysics’ Baoyu Zhao, Pengbiao Xu, Tao Jing and Omkar Shinde; the Department of Chemistry’s Chesley M. Rowlett; and the Department of Microbial Pathogenesis and Immunology’s A. Phillip West and Yuanjiu Lei were involved in the team’s research, which was funded by the National Institutes of Health (Liu Grant No. R01GM121584; Li Grant No. R01AI145287) and The Welch Foundation (Liu Grant No. A-1715; Li Grant No. A-1931). Zhao and Xu were co-first authors on the team’s paper, “The Molecular Basis of Tight Nuclear Tethering and Inactivation of cGAS,” which can be viewed online along with related figures and captions.
To learn more about Liu and his research, visit https://www.chem.tamu.edu/rgroup/liu/.
For more information on Li and his research, go to https://bcbp.tamu.edu/research/faculty/pingwei-li/.
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Contact: Shana K. Hutchins, (979) 862-1237 or shutchins@science.tamu.edu; Dr. Wenshe Ray Liu, (979) 845-1746 or wliu@chem.tamu.edu; or Dr. Pingwei Li, (979) 845-1469 or pingwei@tamu.edu
