Viral infections continue to impose a significant global health burden. Despite ongoing research, the life cycles and pathogenic mechanisms of many viruses remain poorly understood, limiting treatment options and vaccine development. Several recent studies have identified a few unique pathways and proteins involved in the pathogenic response of different viral species, particularly the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway. The cGAS-STING plays a vital role in the virulence of many viruses, including ectromelia virus (a poxvirus) and rhinovirus C (RV-C), and its role varies in response to individual pathogens.
In this review, three research articles are reviewed that discuss the role of chemical mediators in the pathogenesis and life cycle of common viral diseases. In 2008, a protein called stimulator of interferon genes (STING) was identified as the primary stimulator of type I interferon (IFN) in response to foreign DNA. A ligand protein for STING called cyclic guanosine monophosphateadenosine monophosphate (cGAMP) synthesized by “cyclic-GMPAMP synthase” (cGAS), was discovered in 2013 (Wan et al. 2020). For studying Poxviruses, the ectromelia virus (ECTV), which causes mousepox, was used to investigate the impact of the cGAS-STING pathway on its virulence due to its similarities with OPV (Cheng et al., 2018). A unique genotype of RVs, RV-C, was studied in relation to STING due to the poor understanding of its mechanism and relatively severe disease caused by it (Gagliardi et al. 2022).
Cheng et al. (2018) investigated poxviruses; the WT strain of ECTV was obtained and inserted into Vero cells through various plasmids. Transfection was done, and luciferase activity was calculated. RNA samples were isolated, and interference was run with artificially manufactured siRNA. Macrophage samples were obtained from the peritoneal cells of infected mice kept under controlled conditions at specified intervals for ELISA assay and western blot analysis. A PCR was done to calculate DNA copies, and histologic analysis was performed. (Cheng et al. 2018).
Wan et al. (2020) described the cGAS-STING pathway’s mechanism and interactions. The cGAS pathway is activated after detection of B-type foreign dsDNA in the cytoplasm of the cell. This produces cGAMP and causes the digestion of the cGAS-DNA complex. cGAMP can cross cell borders in a number of ways, causing dimerization of STING, leading to increased formation of type-1 IFN. After ubiquitination, cGAS-STING is lysed through autophagosomes. Factors affecting the regulation have been tabulated. This pathway can interact with other DNA-sensing pathways of the cell, having a positive or negative impact on apoptosis. Increased IFN production contributes to cell ageing. cGAS-STING pathway plays a vital role in defence against certain viral (HIV, herpes), bacterial (mycobacterium) and protozoal (T. gondii) diseases. On the other hand, disorders of the cGAS-STING pathway can cause autoimmune diseases like SLE. It can also contribute to certain inflammatory diseases like non-alcoholic steatohepatitis (NASH) and scoliosis.
Certain neurodegenerative disorders, like Parkinson’s disease (PD), are complicated by the apoptotic cascade that this pathway triggers.IFN-1 plays an essential role in regressing tumour cells and limiting their abnormal growth. However, certain research demonstrated STING to act as a double-edged sword by increasing chronic inflammation and contributing to the development of certain tumours. This interaction of cGAS-STING with malignant cells is complicated and depends upon a number of variable factors involved. The practical ability to modulate the cGAS-STING pathway for the treatment of certain diseases was put to the test. It can help by boosting the immunological status on the one hand and by controlling abnormal inflammation on the other hand. The effect of certain anti-tumour drugs, like CD47 blockers, was reduced in mice in the absence of STING. A similar decrease in the effectiveness of certain anticancerous drugs was seen when they were used without a STING agonist. A few STING agonists, including dimethylxanthenone-4-acetic acid (DMXAA), 10-carboxymethyl-9 (10H) acridone (CMA) and amidobenzimidazole, were tested for practical application. Certain drugs like aspirin and suramine were found to have an inhibitory effect on cGAS. The review included information on STING agonists (e.g., DMXAA, CMA, amidobenzimidazole) and inhibitors (e.g., aspirin, suramin) (Wan et al. 2020).
Gagliardi et al. (2022) studied RV-C15 in human airway epithelial (HAE) cultures. HAE cells were taken from four subjects and were placed in in vitro cultures of the human upper airway tract. Different variants of RV were obtained. STING was removed using a variant of CRISPR. Variants of RV were incubated at different temperatures for a fixed period of time, and were quantified through qPCR. Before inoculating RV cells into modified HAE, they were rinsed with 100μl of phosphate-buffered saline (PBS) to make them optimal for use. Cultures were obtained
after the addition of buffers from various chambers and stored for Western blot (WB) analysis at specified temperatures. After numerous rounds of incubation and washing of the apical surface of the culture in a controlled way, fluorescent antibodies were applied along with stains. All the required imaging was taken with a Zeiss Axio Observer 3 Inverted fluorescence microscope, and the same was done for all assays. The calculation of fluorescent levels was done by the Corrected Total Cell Fluorescence (CTCF) method, and the colocalization study was done by the specified method. The method used in this study was based on the issued guidelines of Transmission Electron Microscopy (TEM).
Immunoblotting assays were carried out with primary and secondary antibodies. In order to determine the amount of lactate dehydrogenase produced as a response to RV infection, samples were taken and studied with the Cytotoxicity Assay kit. The Transepithelial Electrical Resistance (TEER) assay was also measured. Junction Analyser Program (JAnaP) was applied to measure the properties of the junctional region. The clearance of mucociliary transport was assessed via transport of 2μm red-fluorescent polystyrene microspheres. (Gagliardi et al. 2022).
Cheng et al. (2018) found that IFN-β production in response to ECTV infection was dependent on both cGAS and STING. ECTV infection was able to produce IFN-1 in certain cells while failing to do so in others. An increase in the production of IFN-β only in the co-presence of STING and cGAS pointed towards the fact that they are essential for its production. It was determined that the cGAS-STING pathway was essential for the making Tbk1 and Irf3 during
ECTV infection. It was observed in knocked-out macrophages, infected by ECTV, that either the levels of IFN were reduced or completely depleted. This signified the driving value of cGas–Sting–Tbk1–Irf3 Pathway in response to viral infections. In vitro experimentation demonstrated the significance of cGAS and STING to limit the replication of the ECTV virus. This pathway was also found to be essential for developing resistance against mousepox in vivo. Even in the presence of TLR9, the absence of cGAS and STING led to an inadequate resistance among mice. (Cheng et al. 2018).
Wan et al. (2020) highlighted the dual role of the cGAS-STING pathway in enhancing immune responses and triggering chronic inflammation. It was concluded that the cGAS-STING pathway performs a binary role in providing immunity. In some cases, it can enhance the immunological response while dampening it at other occasions. It’s pro-inflammatory can enhance the adaptive immunity response transiently and cause cell death in the long term, if needed to protect against harmful inflammatory reactions. It can provide the groundwork for understanding the inflammatory processes involved in neurological diseases. It can have practical application in curative medicine if appropriate drugs are developed to enhance the desirable effects of this pathway and mask the undesirable ones. (Wan et al. 2020).
Gagliardi et al. (2022) determined that RV-C15 replication was associated with ciliated airway cells. The replication of RV-C15 was found to be associated with cells lined by cilia. The shaping of replicative complexes inside the RV was noted, and infection caused the formation of a number of accumulations filled with pockets. The Golgi apparatus serves as the raw material for the formation of complexes during infection, causing a surge in PIP4 levels. Endoplasmic reticulum was found to be the host site for the multiplication of the genome for certain viruses like RV-C15. This virus showed the ability to generate incomplete autophagy because it was unable to inhibit autolysosome production. An increase in the generation of STING was beneficial for the multiplication of the viral genome. RV-C15 was determined to play a pivotal role in changing the permeability of the membrane and the arrangement of proteins present in the junctional zone. (Gagliardi et al. 2022).
The cGAS-STING pathway is a central component of the innate immune response to viral infections. There is a great contradiction in the responses produced by the cGAS-STING pathway to the same genus of pathogens. All these experiments were carried out on non-human subjects, which greatly limits the validity of the results obtained by these trials. While animal and in vitro models provide valuable insights, the translational potential remains uncertain due to variable responses and limited human data. Future studies should focus on confirming safety and efficacy in human systems and exploring targeted modulation of the pathway in clinical settings.
Human testing can only be possible after and only if the safety of its application is confirmed. Moreover, a thorough review is needed to compare the cost-benefit relationship of all these projects. A better impact can be generated if different species are tested, and the results are cross-
referenced for the possibility of human application. Despite current limitations, cGAS-STING remains a promising target for antiviral, anti-inflammatory, and anticancer therapies.