In many living organisms, cytosolic nucleic acid sensing serves as a critical element of immunity. It is a relatively new field of research that involves pathogen detection and insect immune responses to infection. In mammals, the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway is responsible for detecting cytosolic DNA to trigger an effective innate immune response against pathogen infection (1). In this signaling cascade, the binding of cGAS to cytosolic nucleic acid initiates signaling, which stimulates the production of 2′3′ cGAMP, a second messenger protein, and a strong inducer of STING (2–4). Because the cGAS-STING pathway is driven by the presence of nucleic acid rather than pathogen-specific characteristics, it can be activated even when pathogen-specific attributes are not present (5). For this reason, cGAS or cGAS-like molecules of insects can sense a wide range of nucleic acid, both self-origin and foreign. The diverse biological roles of the cGAS-STING pathway in the innate immunity of mammals are comparatively well understood. In contrast, it is an emerging field of study, with its crucial biological roles in host defense becoming clearer as research progresses. Several examples highlight this signaling pathway’s host protective biological role during pathogen infection (6, 7). Recent investigations on the cGAS-STING pathway have revealed that it evolved from an ancient bacterial anti-phage mechanism (8). Emerging evidence shows that the impairment of this extremely diverse innate immune recognition mechanism can disrupt organismal and cellular homeostasis by driving aberrant immunological responses (1). Thus, the investigation is underway or has been undertaken to determine strategies that allow selective regulation of cGAS-STING signaling activity in distinct infection settings (9, 10).

Immune signaling in insects (e.g., Drosophila) and mammals have remarkable similarities. Drosophila Toll, for example, acts as a receptor for the cytokine Spaetzle rather than a PRR, and uses a conserved pathway to activate transcription factors of the NF-κB family (11, 12). Some PGRP receptors recognize diaminopimelic acid-type peptidoglycan from Gram-negative bacteria, activating the IMD pathway, which leads to the activation of another NF-κB transcription factor, Relish (13). Recent studies have shown that in addition to remarkable conservation in immune pathways, mammals and insects exhibit divergent anti-viral immunity, notably cGAS-STING signaling pathway. Thus, in recent years, researchers have become interested in evaluating the relevance of this immune signaling pathway (11, 13). Despite the fact that several studies on various insect species have discussed the role of cGAS-STING in defense against various pathogens, the research is still in its early stages. These studies improved our understanding regarding the cGAS-STING in insects, many controversial events in diverse insects species need to be investigated further.

In this review, we especially focus on recent advances and developments in the cGAS-STING pathway of insects in comparison with mammals. Further, we discuss its biological role in the induction of innate immune responses against pathogen infections. We describe molecular mechanisms that drive cGAS-STING signaling pathway activity and briefly discuss the current state of understanding of signaling by the cGAS-STING signaling pathway. Outline the current state of knowledge about the structure and modulation of the cGAS-STING signaling pathway in insects.

It has recently been shown that cGAS-STING signaling pathways involves in various physiological activities of living organisms and have been suggested to remain conserve throughout the evolutionarily period. Although, the purpose of this review is to discuss the involvement cGAS-STNG pathway in insects. In this we will give a brief over of the cGAS-STING pathway in other animals, which will help to understand the evolution of this signaling cascade.

Insects efficiently produce antimicrobial peptides in response to fungal and bacterial infections. TOLL and IMD, two evolutionary conserved immune signaling pathways, govern the production of these peptides: TOLL, IMD, and JAK-STAT, in addition to RNA interference (RNAi), have recently been reported to be implicated in the host defense against certain viruses. Anti-viral immune responses, on the other hand, rely on RNAi to detect and process intracellular viral double-stranded RNAs. In addition, a large number of genes transcription levels are increased due to viral infection, suggesting an induced immune response against viral infection (47). Invertebrates, such as oysters have been shown to have IFN-like immune responses against viral infection, implying that such a response exists in other invertebrates, including insects (48). The biological role of STING in insect innate immunity has recently been come into the spotlight. Insects’ ability to detect and neutralize virus infections is still unknown. When cytosolic nucleic acid ligands sense viral nucleic acid in mammals, they stimulate STING, which then activates type I interferon and NF-kB immune responses, reducing viral infection (33). Insects, like their mammalian counterpart, also express STING orthologs but how they participate in anti-viral immune responses were obscure.

STING was discovered by a group of scientists who were investigating the molecular mechanisms underpinnings of the innate immune response against Zika virus. Buchon and co-workers found that viral infection activates the IMD pathway activation in the Drosophila brain and that mutant Drosophila for Relish or peptidoglycan recognition protein-LC and -LE, two PRRs that induce the IMD pathway, were more susceptible to viral infection than controls. The authors also noted that Zika virus infection-induced STING expression and that this stimulation was dependent on Relish, suggesting that STING has biological roles as an NF-κB-modulated anti-viral effector (49, 50). In addition, Lamiable et al., followed up on the discovery that several DNA viruses of insects independently hijacked a gene encoding an immunoregulatory cytokine, suppressing the activation of the IMD signaling pathway (51). These findings prompted a thorough examination of IMD pathway’s role in anti-viral innate immunity, which showed that two components of the pathway, including Relish and kinase IKKβ, but not the entire pathway, are required to resist infection by two picorna-like viruses in vivo and a cell line. Another study performed epistasis analysis and Nazo (an anti-viral factor) expression as a readout to explore the link between STING and the IKKβ- and Relish axis. The authors observed that the over-expression of STING resulted in a considerable increase in Nazo the transcript levels. But when IKKβ was suppressed, Nazo expression was equal to that of the control group, implying that STING is potential regulator of Nazo upstream of IKKβ. Of note, STING over-expression was disrupted by the suppression of Relish, IKKβ, and Nazo (anti-viral factor), implying that this process regulates anti-viral gene expression independently of the IMD pathway (41). Later on, Cai and co-workers injected naturally existing CDNs and noted that four of three CDNs induced the STING-regulated genes, which are important in Drosophila’s defense against viral infections. In addition, the authors reported that this protection was entirely based on Drosophila STING and Relish and that ATG7 and AGO2 were not required. Although the biological role of most the STING-regulated genes is unknown, Nazo and Vago have been shown to have a role in anti-viral resistance (41, 52, 53). Interestingly, a study on another insect (Bombyx mori), using a Liquid Chromatography coupled with tandem Mass Spectrometry analysis, confirmed the production of cGAMP in the cytoplasm of BmE cells. They also discovered a human STING homologous protein in this species, which they linked to Relish. The authors noted that infection with nucleopolyhedrovirus (NPV), (a DNA virus from the Baculoviridae family) stimulates the co-expression of STING and Relish protein, and any change in STING concentration affect the synthesis of Relish protein. After binding with Dredd protein, STING activates Relish and promotes its nuclear translocation. Finally, they proposed a conserved cGAMP–STING–NF-kB signaling axis that protects silkworm from NPV infection by increasing antimicrobial peptides (e.g., gloverin and cecropin) production ( Figure 2 ) (40). This study opens up new avenues for research into the anti-viral immune response of insects, and more evidence from other insect species will help to clarify this anti-viral defense mechanism in insects.

Recently, Holleufer et al. (44) used a genetic approach to identify cGAS-like enzymes, including cGLR1 and cGLR2 and concluded that both these proteins respond to viral infection in Drosophila. They noted that an equal amount of these enzymes was expressed in flies and cells cultured in vitro and that this was enough to induce STING. Drosophila with cGLR1 mutations behaved similarly to STING mutants, with a decrease in the activation of various STING-associated genes (Srg1, Srg2, and Srg3), and an increase in infection by Kallithea virus (a DNA virus) and Drosophila C virus (an RNA virus) as well as reduced survival. However, the author noted that while cGLR2 mutant alone does not produce the same results as the cGLR1 mutants, the cGLR2 enzyme has some overlapping biological roles with cGLR1, as evidence by the fact that double-mutant Drosophila lacking both the cGLR1 and cGLR2 genes is more susceptible to infection by Kallithea viruses or Drosophila C virus than a single knockout. In addition, compared to a single knockout showed a higher level of viral replication, a lower survival rate, a more-severe abnormality in the activation of STING-modulated genes, implying that both genes can trigger viral infection signaling and promote fly survival. In addition, natural Drosophila pathogens, the authors investigated the effects of vesicular stomatitis virus (RNA virus) and invertebrate iridescent virus 6 (DNA virus), although neither virus caused any overt phenotype for viral load and survival in cGLRs mutants. Furthermore, these viral infections did not result in a robust STING response, implying that 3′2′-cGAMP and STING are suppressed by a viral molecular mechanism that has yet to be discovered.

The second group of researchers identified cGAS in fly using different research techniques and offered a precise mechanism of STING activation and anti-viral immune responses (43). They discovered that gene CG12970 is a Drosophila cGLRs that is similar in architectural and biological relevance to cGAS and termed it cGLR1 based on the sequence analysis and systematic biochemical screening. The cGLR1 requires double-stranded RNA ligands that are longer than the 21-23-bp RNA molecules commonly produced during RNA interference in Drosophila, indicating that self-recognition is avoided specifically.

Insect cGLRs have a similar ligand detection mechanism to human cGAS, suggesting that insect cGLR1 solely senses a foreign RNA. The authors also reported that Drosophila STING has a highly conserved V-shaped homodimeric architecture with a deep central pocket that binds to 3′2′-cGAMP. The STING–3′2′-cGAMP structure has a tightly ‘closed’ conformation with STING protomers, similar to the closed conformation of human STING bound to 2′3′-cGAMP (38), demonstrating that STING activation is driven by specific 3′2′-cGAMP-dependent signaling. Finally, they show that D. melanogaster, a cGLR-STING-NF-κB axis protects animals from viral replication by activating the synthesis of the anti-viral genes (STING regulated genes).

Altogether, insect STING modulates NF-κB-dependent immune responses and is involved in anti-viral innate immunity. The discovery of genes encoding cGAMP in the genomes of insect (dipteran, lepidopteran) hosts suggests that cGAS-STING signaling plays an important role in insects anti-viral innate immunity. So far, different anti-viral factors, including antimicrobial peptides, have been reported; however, further evidence is required to fully understand the anti-viral responses induced by cGAS-STING in insects.

Autophagy is an ancient biological process that is involved in a variety of physiological activities, such as immune responses, development, and aging. Autophagy also involves in degrading aggregated or misfolded proteins, eliminating damaged cellular organelles, including endoplasmic reticulum, mitochondria peroxisomes, and also abolishing intracellular microbial pathogens (54). This process starts with various stressors and leads to the targeted isolation of cytoplasmic contents within autophagosomes, which then merge with lysosomes to break down the engulfed cargo. Autophagy has been implicated in the host immunological defense against a variety of intracellular microbial pathogens in insects and other animals. Upon the pathogen infection, innate immune signaling induces autophagy, which degrades intracellular viruses, bacteria, and parasites (55). Thus, autophagy has emerged as a crucial biological process for suppressing pathogen replication or pathogenicity and host survival.

Zika virus, an arthropod-borne virus, infection leads to severe complications as neuroprogenitors are lytically infected. Like other arboviruses (e.g., West Nile virus), which infect the central nervous system of mosquitos and are regulated by poorly understood immune mechanisms, Zika virus particularly infects and replicates in adult Drosophila brains (56, 57). Liu et al. (55), explored innate immune mechanisms that suppress the Zika virus infection in the fly brain. They discovered that the anti-viral RNA interference pathway is not involved in the suppression of Zika virus replication, implying that the virus encodes an RNA interference silencing suppressor. However, because Drosophila mutant for Relish/NF-kB transcription factor had a greater viral infection, the authors concluded that Zika virus infection in the fly brain promotes the Relish/NF-kB/IMD pathway, which is implicated in viral infection suppression. This canonical Releish/NF-kB pathway is stimulated by the pattern recognition receptors, peptidoglycan recognition proteins-LE or peptidoglycan recognition proteins-LC, which interact with bacterial peptidoglycans either intracellularly or extracellularly, respectively ( Figure 3 ). The authors observed that flies mutant with both of the above-mentioned receptors are more vulnerable to Zika virus infection. However, the authors were unable to determine whether these receptors bind to the Zika virus directly or indirectly to trigger this protective cascade. In addition, they found a Rel/NF-KB-dependent induction of STING in the fly brain in response to Zika virus infection and the fact that STING is a downstream factor of Rel/NF-KB, implying that STING is involved in the brain’s anti-viral immune response. Because this signaling cascade in insects does not activate IRF3-dependent type I interferons signaling, the authors assumed that STING is likely to govern anti-viral autophagy. They supported this hypothesis by examining autophagy-related factors (Atg5, Atg7, Atg8-II, and mCHERRY-Atg8 puncta), and they concluded that autophagy is a critical process controlling Zika virus replication because they observed an increase in the Zika virus replication after autophagy-related factors were lost (55). The molecular mechanism by which STING activates autophagy, and the viral pathogens cargo target for degradation remains unknown. In contrast, a recent study generated ATG7 mutant Drosophila and injected them with Drosophila C virus. Interestingly, the authors argue that 2′3′-cGAMP, along with Drosophila STING and Relish, regulate viral infection independently of the canonical autophagy pathway route, resulting in a reduction of viral infection (53). Based on these studies, it appears that the host may have a virus-specific role or that an unconventional autophagy pathway is induced to eliminate a viral infection. However, more research is required to fully understand STING-dependent autophagy in insects. Recently in another study, suggested that silkworm (Bombyx mori) innate immune responses (autophagy) against parasite, Nosema bombycis infection are STING-dependent comparable to those observed in vertebrates (58). The authors reported that B. mori ATG8, an autophagy-related factor, was up-regulated after N. bombycis infection. In addition, after N. bombycis infection, they noted that the level of LC3 (biochemical hallmark of autophagy) in the midgut of BmSTINGΔ6bp/WT, and BmSTINGΔ5bp/WT (STING knockout transgenic lines) is lower than that of wild type B. mori (40), suggesting that loss of STING is linked with LC3. In insects, this change in the level of autophagy-related protein suggests that STING control autophagy LC3 protein. However, in this study, at the latter stage of infections, all of the transgenic lines infected with N. bombycis died. This phenomenon raises several questions about the possible causes of larval death during the latter stages of infection, such as whether the host’s resistance mechanism is insufficient to completely remove the parasite or whether the parasite has managed to hijack autophagy and redirect this process to support their replication within the host (59, 60). Hua et al. (40), investigated the possible mechanism of the parasite to take over host immunity. In their study, the authors found that N. bombycis infection stimulates silkworm protein degradation, which in turn supports in the synthesis of host ATP, which is necessary for delivering nutrients and energy to invading pathogens (61). A detailed molecular mechanism is needed to explore to understand the resistance mechanism of the host against the parasites. However, this study opens new research avenues to investigate how insects respond to parasitic pathogens. The ERGIC is formed in mammals when STING binds to cGAMP interacts with SEC24C, which causes the endoplasmic reticulum to produce COP-II vesicles, which eventually form the ERGIC. The ERGIC serves as a membrane source for WIPI2 recruitment and LC3 lipidation, which results in the production of autophagosomes that target cytosolic pathogens for degradation by the lysosome. Of note, the stimulation of LC3 in response to cGAMP is the characteristic feature of mammalian STING and is due to the presence of TBK1 activation domain at the C-terminus of the protein. It has been shown that the C-terminal architecture of insects STING, especially the Bombyx mori STING, is highly conserved with human and mouse STINGs (40). Thus, it seems that insect STINGs may retain the ability to induce autophagy response as a result of the conserved architecture at the C-terminus. In addition, although STING in some vertebrates, such as sea anemone, lacks the TBK1 activation domain, it is still capable of stimulating LC3 in response to cGAMP stimulation (31). Thus, autophagy stimulation is an ancient and highly conserved biological role of the cGAS-STING signaling pathway in animals (46).

Overall, it seems that STING-dependent autophagy is a conserved physiological process in insects, in addition to the production of anti-pathogen factors to eliminate microbial infection similar to those of vertebrates counterparts (62, 63). However, many questions like most of the STING-dependent autophagy factors and their mechanism of function is unknown. It is also a matter of interest whether induction of autophagy in insect host is a general mechanism against different pathogens, e.g., viruses, or this process only activated against specific viruses. Experimental evidences are also required to ensure whether insect host induces STING-dependent autophagy in addition to the synthesis of antimicrobial factors. Future studies will help to understand these questions.

So far, most of the studies addressed the role of cGAS-STING signaling in the host defense against viruses. In some studies, it has been reported that STING plays a crucial role in the prevention of bacterial infection. Martin and co-workers were the first to report STING mutant Drosophila had dysregulation of the IMD pathway, which resulted in reduced resistance by the intracellular bacterium Listeria monocytogenes. In this study, the authors demonstrated that CG7194, the closest homolog to cGAS in flies, did not cause any change in mortality against L. monocytogenes infection, suggesting that CG7194 (cGAS homolog) is not involved in activating STING signaling in bacterial infection and that signaling could be induced by the bacterial CDN 3′3′c-di-GMP. It appears from these results that in Drosophila, STING functions as a direct sensor of bacteria through binding of CDNs and activates an IMD- and Relish-dependent anti-bacterial response (42). They went on to demonstrate that STING can bind to c-di-GMP, and that a mutant protein product lacking a CDN-binding domain can completely abolish this binding. While Drosophila STING is structurally varying to mammalian STING, as it lacks the CTT, which is an essential region for monitoring downstream signaling transduction to IRF3. Mutants of mammalian STING that lack CTT are unable to activate IRF3, although this mutant has the ability to activate NF-κB signaling. Likewise, Drosophila STING has the ability to stimulate NF-κB signaling. After infection with L. monocytogenes, the release of c-di-GMP promotes the Drosophila IMD signaling pathway, resulting in increased expression of IMD-related antimicrobial peptides (AttacinA and CecropinA2). It is possible that STING operates through Relish to activate the IMD pathway, as evidenced by the reduction in cleavage and activation of Relish in STING knockout flies. In addition, the authors ruled out the possibility of involvement of the Toll signaling pathway because they did not observe any change in the Toll-related antimicrobial peptides (drosomycin) during the experiment. Infection with L. monocytogenes, STING-overexpressing flies have a lower bacterial load, lower mortality, and decreased susceptibility to the infection (42). In addition, the AttacinA and CecropinA2 genes have highly expressed these flies. The depletion of Relish or IMD in Drosophila STING-overexpressing flies results in a reduction in the production of antimicrobial. Interestingly, the Drosophila CG7194 putative cGAS homolog lacks a zinc ribbon domain and a positively charged N-terminus, both of which are required for DNA binding (4). The depletion of Drosophila CG7194 has no effect on the mortality rate of insects infected with L. monocytogenes, indicating that Drosophila CG7194 may not be involved in innate immune responses against bacterial pathogens (42). The overall conclusion is that Drosophila STING directly senses bacterial CDNs, especially c-di-GMP, and triggers anti-bacterial immunity via IMD-Relish signaling in the absence of a functional cGAS ortholog.

In contrast, a previous study reported that the Toll and IMD signaling pathways in D. melanogaster were activated in response to the infection with L. monocytogenes. The authors demonstrated that dif or IMD mutant flies are more susceptible to L. monocytogenes infection, suggesting that this bacterial infection can promote the induction of Toll and IMD pathway in the fly (64). Another study conducted with a bacterial pathogen, L. monocytogenes in Tenebrio molitor, recently saw a similar outcome (significant increase in IMD and Toll specific antimicrobial peptides) with the same conclusion. The authors suggested that, in addition to antimicrobial peptides, Relish regulates the autophagy-related proteins (e.g., serine-threonine protein kinase), since they saw significant downregulation in autophagy-related proteins T. molitor that was deficient in Relish expression (65). The induction of IMD, Toll signaling pathways, and autophagy in response to L. monocytogenes infection suggest that there may be possible cross-talk between Toll and IMD signaling and autophagy.

On the whole, based on the findings discussed above, Relish appears to be a key regulator of different signaling pathways, including Toll, IMD, and autophagy. Because of the variations in these results, it is possible that there is a -crosstalk between Toll, IMD and autophagy processes. On the other hand, Kranzusch et al. (31) reported the binding of CDNs to STING from the N. vectensis and supported the hypothesis that the ancestral biological role of STING in metazoans was to recognize CDNs. Bacteria synthesize different CDNs and cyclic trinucleotides, some of which are capable of activating the Drosophila STING gene (66). However, in contrast to Martin et al. (37), who suggested that bacterial c-di-GMP could induce a STING-dependent immune response to L. monocytogenes infection. A recent study found no evidence of a contribution of STING or induction of antimicrobial peptides in response bacterial c-di-GMP released after L. monocytogenes infection (53). Thus, further studies are required to clarify these discrepancies by considering other parameters such as species variations, insects microbiome, etc.

Cytosolic RNA or DNA has been identified as a possible immune inducer that has the potential to activate a robust innate immune response against pathogens for the host defense (67–70). It is the STING that is responsible for the induction of innate immune responses by sensing cytosolic RNA or DNA (32, 71–76). Furthermore, if the STING is not regulated, the persistent activation of immune responses triggered by RNA or DNA could results in uncontrolled innate immune responses that could have an impact on physiological process of the host. Thus, it appears to be essential that there would be a mechanism to keep the effect of persistent activation of STING in a cell under control. In mammals, several molecular mechanisms have been identified that reduce STING activity, such as accelerated degradation by TRIM30α (77), ULK1-mediated phosphorylation (78), impediment of its interaction with TBK1 by NLRC3 (79), RNF5-mediated ubiquitination and degradation (80) and so on. Because of the limited number of studies that has been done on this molecular mechanism of immune responses in insects, only a few STING regulators have been identified in different insect species in recent years.

Roquin is a novel RING-type ubiquitin ligase family member that has recently been discovered in insects. These proteins control the production of T-lymphocytes in vertebrates, and loss of their activity can cause an imbalance of T-lymphocyte (81). The proteins are extremely well conserved, and they have a novel ROQ domain that is resided on the N-terminus of the protein (82). A recent study reported that this protein exists in Drosophila and that it is linked with STING regulation (83). The STING regulators in Drosophila were identified after screening nine genes encoding the ubiquitin ligase in S2 cells in vitro and analyzing their effect on STING-dependent immune responses. The authors reported a negative correlation between roquin protein and STING signaling. In addition, over-expressing of roquin in S2 cells has been shown to inhibit STING-dependent immune response. In contrast, roquin depletion resulted in uncontrolled production of antimicrobial peptides and the reduction of the replication of L. monocytogenes (83), suggesting that roquin is a potential negative regulator of STING-dependent innate immune response in insects. It has been shown that the ROQ domain of roquin is flanked by a RING and CCCH finger motif (84). It is still unclear what biological roles the RING-1 zinc finger of roquin performs; however, future studies may shed light on its involvement in the regulation of STING activity. However, it has been demonstrated that proteasome and Polyubiquitylation activity are required for rapid breakdown or degradation of mRNAs that contain destabilizing ARE elements in their 3’-untranslated regions (85). Thus, it appears that roquin may stimulate the ubiquitin-mediated degradation of proteins that control mRNA stability or that ubiquitin-tagging may alter the association or localization of mRNA modulating proteins.

In addition to the Roquin protein, another study suggested the interaction of Caspase 8L with STING protein based on the in vivo and in vitro experiments. In comparison to Dredd, the caspase 8L protein shares 70% identity with the N-terminus domain but does not include the C-terminus caspase domain, which is implicated in the cleavage of Relish, implying that while caspase 8L may be involved in Relish processing, it may perform the different biological role (40). In BmE cells, over-expression of Caspase 8L together with Relish has been shown to repress Relish after cGAMP or BmNPV stimulation in the cells. In vitro, a caspase 8L deficiency results in a high level of resistance to viral infection, suggesting that caspase 8L is involved in negatively regulating STING-dependent signaling in silkworm, Bombyx mori, and possibly in other insect species (40). However, the precise participation of Caspase 8L to the regulation of STING-dependent signaling needs to be determined in more detail in insects ( Figure 4 ).

All of these results show that roquin and caspase are involved in the negative regulation of virally induced STING-dependent signaling and innate immunity. Although these studies data demonstrating the involvement of certain proteins in the modulation of STING cascade; nevertheless, the exact molecular mechanisms through these proteins contribute to the negative control of STING-dependent signaling remain still an unanswered questions.

In order to complete their life cycle within the host, microbial pathogens may devise strategies to evade the host immune system (86–90). Thus, in order for pathogens to successfully commence replication, they must prevent or inhibit activation of nucleic acid sensors (cGAS or cGAS-like). In mammals, several studies have highlighted the mechanism by which pathogens control the host immunity. For example, DNA viruses (such as retroviruses and herpesviruses) shield DNA within the viral capsid so that it does not become detected by cytosolic nucleic acid sensors until it reaches the nucleus (91–97). The activation of STING by BmNPV (DNA virus) infection was recently described by Hua et al. (40), and it is likely that baculoviruses may also use the same strategy to bypass the host’s immune system.

Another strategy to restrict activation of cGAS or cGAS-like is to target cGAS or cGAS-like for degradation, decreasing levels of this nucleic acid sensor and reducing 2′3′-cGAMP production. This is accomplished in numerous ways by both RNA and DNA viruses. In mammals, poxvirus immune nuclease (poxin) is involved in the degradation of 2′3′-cGAMP and also blocks the stimulation of anti-viral immune signaling through the STING signaling cascade during infection of poxvirus. The poxin of most poxviruses occurs as a fusion to a C-terminal domain with homology to mammalian schlafens (5, 98, 99). The enzymes also have also been reported in insects, and it seems to have a prominent function for 2′3′-cGAMP signaling in insects. Several studies separately documented the STING signaling pathway in insects that drives NF-kB and autophagy signaling to prevent viral infection (40, 41, 46). Recent work on insects involves a cGAS-like molecule to activate STING signaling (43, 44, 100). It seems that poxin enzymes of insect viruses likely prevent activation of immune response comparable to the biological role of poxin of mammalian poxviruses.

In summary, since our understanding is increasing regarding cGAS–STING signaling insects, nevertheless bacterial and viral components that may target this signaling pathway for the successful replication need to be elucidated. So far, our knowledge is rather preliminary regarding the negative or positive regulation of this pathway. Therefore, future studies are required to determine how microbial pathogen suppresses the STING immune signaling activation in insects. In addition, how insect viruses poxin sense the cGAS-like molecules at a molecular level and how microbial pathogen escape, shape the structure and modulation of STING-dependent immune pathways in different insect species.

The biological roles of the cGAS-STNG signaling pathway in the immune system of mammals have been well-established. In contrast, this pathway has recently been discovered in insects, and biochemical evidences supports the immunological functions of the cGAS-STING pathway in insects. The lack of extensive literature is limiting for any direct and clear conclusions. But, with this research background, we endeavored to review available knowledge and develop preliminary conclusions based on our understanding of the subject. Furthermore, research on insects cGAS-STING is still in its early stages, and more investigations are needed before a definite conclusion can be drawn on this topic.

The authors’ responsibilities were as follows: LD, MNA, and BT designed this review article. QL, SK, YT, and WH downloaded material and write down draft. QL, SK, and MNA draw the diagram and proofread the article. All authors contributed to the article and approved the submitted version.

This work was funded by the Natural Science Research General Program of Jiangsu Provincial Higher Education Institutions (21KJA240003), the Natural Science Foundation of Jiangsu Province (BE2020673), the Doctorial Start-up Fund of Southwest University (SWU020023), the National Key R&D Program of China (2019YFD0900404-05), the Industry-University-Research Cooperation Project of Jiangsu Province (BY2020644), the Open Funding Project of Anhui Province Key Laboratory of Aquaculture & Stock Enhancement (AHSC202001), the 16th Six Talents Peak Project of Jiangsu Province (NY-126), the National Natural Science Foundation of China (32070526), the Jiangsu Agriculture Science and Technology Innovation Fund (CX(18)3027). The study was sponsored by the Qing Lan Project of Jiangsu Province, and the “Outstanding Young Talents” of YCTU.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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