cGAS is a cytosolic DNA sensor that activates innate immune responses through production of the second messenger 2’3’-cGAMP, which activates the adaptor STING. cGAS senses dsDNA in a length-dependent but sequence-independent manner, and it cannot discriminate self-DNA from foreign DNA. In normal physiological conditions, cellular DNA is sequestered in the nucleus by nuclear envelope and in mitochondria by mitochondrial membrane. When self-DNA leaks into the cytosol during cellular stress, or during mitosis, the cGAS can be exposed to self-DNA and be activated. Recently, many studies have investigated how cGAS keeps inactive and avoids to be aberrantly activated by self-DNA. Thus, the aim of this narrative review is to summarize the mechanisms by which cGAS avoids to sense self-DNA under normal physiological conditions.

The evolutionary conserved STING-cGAS pathway represents one of the most important cytosolic DNA sensing systems that is activated in response to viral invasion and/or damaged integrity of the nuclear envelope. The key outcome of this pathway is the production of IFN, which subsequently stimulates hundreds of genes. In oncology, the situation is a complex one since this pathway may serve either anti- or pro-oncogenic roles in a context-dependent fashion. The prevailing understanding is that when the innate immune response is activated by sensing cytosolic DNA, such as from ruptured micronuclei, it results in the production of interferon which attracts cytotoxic cells to destroy tumors. However, in tumor cells that have adjusted to significant chromosomal instability, particularly in relapsed, treatment-resistant cancers, the cGAS–STING pathway often supports cancer progression. In these cells, the pathway initiates a non-canonical nuclear factor κB transcriptional response, fostering the epithelial-to-mesenchymal transition (EMT). Here, we review this intricate pathway in conjunction with cancer progression and development of new treatment modalities.

The innate immune system is the first line of host defense sensing viral infection. Manganese (Mn) has recently been found to be involved in the activation of innate immune DNA sensing cGAS-STING pathway and subsequent anti-DNA virus function. However, whether Mn2+ mediates host defense against RNA viruses is still unclear. In the current study, we demonstrated that Mn2+ exhibited antiviral effects against various animal and human viruses including RNA viruses such as PRRSVs and the VSV as well as a DNA virus such as the HSV1 in a dose-dependent manner. Moreover, cGAS and STING were both investigated in the Mn2+ mediated antiviral roles using the knockout cells made by CRISPR-Cas9 approach. Unexpectedly, the results revealed that neither cGAS knockout nor STING knockout had any effect on Mn2+ mediated antiviral functions. Nevertheless, we verified that Mn2+ promoted the activation of cGAS-STING signaling pathway. These findings suggest that Mn2+ has broad spectrum antiviral activities in a cGAS-STING pathway independent manner. This study also provides significant insights into redundant mechanisms participating in the Mn2+ antiviral functions and also indicates new target for Mn2+ antiviral therapeutics.

Human Perrault syndrome (PRLTS) is defined by autosomal recessive inheritance with primary ovarian insufficiency and early hearing loss. Most PRLTS disease proteins modulate mitochondrial transcription or translation. Among the genetic causes are ClpP mutations, which trigger also complete azoospermia, whose cellular and molecular underpinnings are unknown. Here, the ClpP-null mouse model was studied by global transcriptomics, proteomics, RT-qPCR, immunoblots, tissue fractionation, testis histology, and was crossed with STING/IFNAR mutants. Spermatogenesis showed accumulated early spermatocytes, versus deficits of desynapsis and kinetochore factors; excess Dazl/Stra8 and acetylSMC3, versus deficient SHCBP1L, were molecular correlates. Spermiogenesis showed few round spermatids, tsHMG/TFAM in elongated spermatids was absent; transcripts for tail/acrosome factors were downregulated from start. Nuclear anomalies included a failed Rec8 induction, early BRDT deficiency, histone H3 cleavage, and cGAMP increase, among antiviral responses typical of ClpP-mutants. However, deletion of downstream innate immune signals STING/IFNAR failed to reestablish fertility. As mitochondrial triggers, we observed accumulation of ClpX, with PTCD1, POLDIP2, GRSF1, ALKBH7, DNAJA3, AURKAIP1, VWA8, and Perrault proteins ERAL1, PEO1, HARS2, partially showing nuclear redistribution. ClpP-depletion is known to cause extra-mitochondrial release of mispacked mtDNA/mtRNA/protein complexes. Now we define nuclear inflammatory responses and meiotic arrest as consequences, similar to observations in mito-mice and mutator-mice.

Human Perrault syndrome (PRLTS) is defined by autosomal recessive inheritance with primary ovarian insufficiency and early hearing loss. Most PRLTS disease proteins modulate mitochondrial transcription or translation. Among the genetic causes are ClpP mutations, which trigger also complete azoospermia, whose cellular and molecular underpinnings are unknown. Here, the ClpP-null mouse model was studied by global transcriptomics, proteomics, RT-qPCR, immunoblots, tissue fractionation, testis histology, and was crossed with STING/IFNAR mutants. Spermatogenesis showed accumulated early spermatocytes, versus deficits of desynapsis and kinetochore factors; excess Dazl/Stra8 and acetylSMC3, versus deficient SHCBP1L, were molecular correlates. Spermiogenesis showed few round spermatids, tsHMG/TFAM in elongated spermatids was absent; transcripts for tail/acrosome factors were downregulated from start. Nuclear anomalies included a failed Rec8 induction, early BRDT deficiency, histone H3 cleavage, and cGAMP increase, among antiviral responses typical of ClpP-mutants. However, deletion of downstream innate immune signals STING/IFNAR failed to reestablish fertility. As mitochondrial triggers, we observed accumulation of ClpX, with PTCD1, POLDIP2, GRSF1, ALKBH7, DNAJA3, AURKAIP1, VWA8, and Perrault proteins ERAL1, PEO1, HARS2, partially showing nuclear redistribution. ClpP-depletion is known to cause extra-mitochondrial release of mispacked mtDNA/mtRNA/protein complexes. Now we define nuclear inflammatory responses and meiotic arrest as consequences, similar to observations in mito-mice and mutator-mice.

Endothelial cells form a powerful interface between tissues and immune cells. In fact, one of the underappreciated roles of endothelial cells is to orchestrate immune attention to specific sites. Tumor endothelial cells have a unique ability to dampen the immune responses and thereby maintain an immunosuppressive microenvironment. Recent approaches to trigger immune responses in cancers have focused on activating nucleic acid sensors such as cGAS/STING in combination with immunotherapies. In this review, we present a case for targeting nucleic acid sensing pathways within the tumor vasculature to invigorate tumor immune responses. We introduce two specific nucleic acid sensors, the DNA sensor TREX1 and the RNA sensor RIG-I and discuss their functional roles in the vasculature. Finally, we present perspectives on how these nucleic acid sensors in the tumor endothelium can be targeted in an antiangiogenic and immune activation context. We believe understanding the role of nucleic acid sensing in the tumor vasculature can enhance our ability to design more effective therapies targeting the tumor microenvironment.

Mitochondrial dysfunctions, e.g. abnormal handling of mitochondrial DNA in TFAM mutants or in altered mitophagy, activate innate immunity. Recent reports also showed that deletion of mitochondrial matrix peptidase ClpP in mice transcriptionally upregulates inflammatory factors. Here, we studied ClpP-null mouse brain at two ages and embryonal fibroblasts, to identify which signaling pathways are responsible, employing mass spectrometry, immunoblots, and reverse transcriptase polymerase chain reaction. Anomalies in the mitochondrial unfolded protein responses pathway were prominent for the co-chaperone DNAJA3, and for its known interactor STAT1. Their mitochondrial dysregulation affected also their extra-mitochondrial abundance, as possible innate immune modulators. Increased expression was observed not only for the transcription factors Stat1/2, but also for two interferon-stimulated genes (Ifi44, Gbp3). Inflammatory responses were strongest for RLR pattern recognition receptors (Ddx58, Ifih1, Oasl2, Trim25) and several cytosolic nucleic acid sensors (Ifit1, Ifit3, Oas1b, Ifi204, Mnda). They can be explained by the accumulation of mitoribosomes and mitochondrial nucleoids in ClpP-null cells, which may act as damage-associated molecular patterns. The consistent dysregulation of these factors from early age might influence also human Perrault syndrome, where ClpP loss-of-function leads to early infertility and deafness, with subsequent widespread neurodegeneration.

Inflammatory bowel disease (IBD), including Crohn’s Disease (CD) and Ulcerative Colitis (UC) are chronic multifactorial disorders which affect the gastrointestinal tract with variable extent. Despite extensive research, their etiology and exact pathogenesis are still unknown. Cell-free DNAs (cfDNAs) are defined as any DNA fragments which are free from origin cell and able to circulate into bloodstream with or without microvescicles. CfDNAs are now being increasingly studied in different human diseases, like cancer or inflammatory diseases. However, to date it is unclear how IBD etiology is linked to cfDNAs in plasma. Extrachromosomal circular DNA (eccDNA) are non-plasmidic, nuclear, circular and closed DNA molecules found in all eukaryotes tested. CfDNAs appear to play an important role in autoimmune diseases, inflammatory processes, and cancer; recently, interest has also grown in IBD, and their role in the pathogenesis of IBD has been suggested. We now suggest that eccDNAs also plays a role in IBD. In this review we have comprehensively collected available knowledge in literature regarding cfDNA, eccDNA, and structures involving them such as neutrophil extracellular traps and exosomes, and their role in IBD. Finally, we focused on old and novel potential molecular therapies and drug delivery systems, such as nanoparticles, for IBD treatment.