In higher organisms, detection of DNA in the cytoplasm leads to an immune reaction. The enzyme that senses DNA is also “out of place” in the nucleus, but DNA has no such effect. LMU researchers are now reporting why.
Structure of cGAS (red) cryo-electron microscopy bound to histone (blue) proteins of the nucleosome. Photo: K.-P. Hopfner
The bulk of DNA in cells of higher organisms is confined to the nucleus, while all other organic DNA is restricted to specific intracellular compartments in the cytoplasm. Thus the appearance of DNA in the soluble phase of the cytoplasm is interpreted by the innate immune system as an indication of the presence of pathogens within cells – usually bacteria or viruses, although cancer cells and senescent cells can also release DNA or mitochondria into the cytosol. . Misplaced DNA – whether nuclear, mitochondrial, or extracellular in origin – triggers a strong immune reaction, which is initiated by the cGAS enzyme. Researchers have long assumed that cGAS itself is localized exclusively in the cytosol. However, recent studies have shown that the protein is actually present preferentially in the cell nucleus. This finding naturally raises the question of what prevents cGAS from binding to DNA and triggering an autoimmune reaction. A team of scientists at the LMU Gene Center led by Professor Carl Peter Hopfner, in collaboration with Professor Vet Hornung and colleagues, showed that the nature of the interaction of cGAS with chromosomal DNA in the nucleus explains why the interaction failed. To stimulate the innate immune system. The new results appear in the leading journal Nature.
When binding to cellular DNA, cGAS makes a messenger molecule that triggers an intracellular signaling chain that leads to the production of proteins that mediate an inflammatory reaction. This process is necessary to eliminate infectious pathogens. However, it is also implicated in the development of autoimmune diseases – some of which actually involve the production of antibodies directed against a cell’s DNA. Thus the fact that cGAS occurs in the nucleus conflicts with the protective function of the innate immune system, since activation of the enzyme in the nucleus itself would be expected to trigger autoimmune reactions against the DNA itself. “Oddly enough, recent data actually indicate that the tight binding of cGAS to the nucleus DNA protein complex – known as chromatin – is critical for the prevention of DNA-based autoimmunity,” Hopfner says.
In the chromatin complex, DNA is wrapped around disk-like molecules and is made up of proteins called essential histones. The resulting “nucleosomes” are linked by “binding DNA” which is not directly related to the basic histones. By cryo-electron microscopy, Hopfner and colleagues were able to show that cGAS binds exclusively to the protein component of chromatin, and does not interact with the DNA itself. “That was a big surprise,” says co-lead author Carina de Oliveira Mann. Moreover, its binding method ensures blockage of the cGAS DNA recognition site. As a result, the enzyme becomes inactive in the nucleus, even when the DNA in its vicinity becomes available to other proteins during gene activation. Ironically, this means that by trapping the enzyme in an inactive state, chromatin actually acts as a reservoir for cGAS.
In fact, cGAS is most effectively inhibited in the least bundle regions of chromatin, where most of the genes are located. “This could explain why cGAS is activated in what is known as the micronucleus in the cytosol, where chromatin is thought to be densely packed,” Hopfner says. The tiny nuclei are composed of chromosome fragments enclosed in a nuclear envelope. They are the product of errors in chromosome segregation in rapidly growing cancer cells or DNA damage caused by ionizing radiation. “Our study represents an important step forward in our understanding of how cGAS interacts with chromatin, and it will help us clarify the inflammatory reaction initiated by the enzyme in the context of cancers and autoimmune diseases,” Hopfner says.
Nature 2020
