Regulatory mechanisms for nuclear cGAS - Project summary Excessive DNA from either the pathogen or host activates the cytosolic DNA sensor, cyclic GMP-AMP synthase (cGAS), which produces the second messenger, cyclic GMP-AMP (cGAMP), initiating a signal cascade leading to the expression of type I interferon (IFN). Although initially considered a cytosolic protein, recent studies reveal abundant expression of cGAS in the nucleus, tethered to chromatin throughout the cell cycle. Despite the nucleosome binding model clarifying the inactivity of nuclear cGAS, many unresolved questions persist. Foremost among them is the crucial query regarding the immunological significance of nuclear cGAS in the DNA-rich environment. Furthermore, the mechanisms underlying the entwining of cGAS into chromatin and the regulation of this process remain unclear. The possibility of nuclear cGAS activation poses another unanswered question, prompting exploration into how and under what physiological or pathological conditions this might occur. Lastly, the activation of cGAS in micronuclei originating from the nucleus is yet to be fully elucidated. Thus, this application is to define the role of nuclear cGAS in the DNA sensing signaling pathway and elucidate the regulatory mechanisms of nuclear cGAS. Based on the existing literature and our preliminary data, we hypothesize that nuclear cGAS is an immune sensor for heterochromatin perturbation. We will investigate three overarching aims. Aim 1 will determine the mechanisms that control cGAS nuclear localization and activation. Aim 2 will determine the mechanisms by which nuclear cGAS senses DNA viruses. Aim 3 will determine the mechanisms controlling nuclear cGAS activation upon DNA damage. Our model proposes that nuclear cGAS is the sensor that elicits an immune alarm by detecting the dangers of heterochromatin perturbations caused by various cell insults, such as viral infection and genotoxic stress. If our hypothesis is correct, this new model will bridge the gaps in understanding the biological significance of nuclear cGAS. Furthermore, such knowledge can be harnessed to rationally intervene in cGAS signaling to boost or dampen innate immune responses in infectious diseases, autoinflammatory diseases, and cancers.
