Journey to Nucleotide Wonderland: Navigating the Adaptability of the cGAS-STING Pathway

In the tireless battle against infections, the intricate molecular strategies employed by our innate immune system often find parallels in various organisms, from invertebrates to plants and even bacteria. Key players in this complex defense mechanism are cyclic GMP-AMP synthase (cGAS), renowned for detecting DNA in the cytosol — a telltale sign of infection, and its potent signaling nucleotide: 2’3’-cGAMP. During my recent lab rotation, I learned about the versatility of the evolutionary ancient cGAS-STING signaling axis by exploring the antiviral potential of recently discovered nucleotides from diverse species in the human context.

Understanding the cGAS-STING Signaling Axis

The cGAS-STING pathway constitutes a pivotal component of the innate immune system, serving as a guardian against viral infections and cellular anomalies. At its core is the cyclic GMP-AMP synthase (cGAS), a sensor strategically positioned within the cell to detect the presence of foreign DNA, typically a sign of viral intrusion or cellular damage. Upon recognition, cGAS, a nucleotidyltransferase (NTase), catalyzes the synthesis of 2’3’-cyclic GMP-AMP (2’3’-cGAMP), a critical second messenger. This molecular cue then engages the Stimulator of Interferon Genes (STING), initiating a cascade of events that culminate in the activation of transcription factors responsible for producing antiviral interferons and other immune mediators.

Beyond cGAS: Exploring Receptor Diversity.

From the fruit fly to coral species to humans, the cGAS-STING axis showcases its adaptability and the diversity of its functions. The comprehensive analysis by Kranzusch and colleagues introduces a new layer to our comprehension with the discovery of over 3,000 cGAS-like receptors (cGLRs) across the animal kingdom. While most animals encode only a few cGLRs in their genome, the study unveils a vast expansion of the gene family in some marine invertebrates. For example, oysters possess over 200 cGLR genes. Structural analyses reveal that many cGLRs can bind either dsDNA or dsRNA, while a subset lacks predicted binding surfaces for nucleic acid ligands, hinting at potential alternative detection mechanisms.

New Nucleotides Enter the Chat in Cellular Signaling

Besides the exploration of novel cGLRs, the authors also present groundbreaking insights into the realm of second messenger molecules, unraveling unexpected findings that challenge conventional wisdom. Spanning from thin-layer chromatography to mass spectrometry analysis, they reveal that while many tested cGLRs produce the familiar 2’3’-cGAMP, a subset surprises researchers by generating cyclic di-AMP (cdiAMP) or pyrimidine-containing compounds, such as cyclic UMP-AMP (cUAMP), with diverse linkage patterns.

What truly drew my curiosity was the structural revelation of human STING bound to 2’3’-cUAMP, as identified in oysters, along with the injection experiments involving 2’3’-cUAMP or 3’3’-cUAMP into live oysters and corals:

• Surprisingly, 2’3’-cUAMP exhibits a binding pattern to human STING that is nearly identical to that of 2’3’-cGAMP.
• Injection of 2’3’-cUAMP into oysters revealed a cascade of responses, including the upregulation of pattern recognition receptors, cytokines, and homologs of anti-viral proteins. This orchestrated transcriptional reaction remarkably mirrors mammals' type I interferon response.

These discoveries challenge traditional beliefs by demonstrating that STING, long associated with purine-containing nucleotides, can potentially be activated by pyrimidine-containing counterparts. This newfound understanding adds a layer of complexity to STING activation, reshaping our perceptions of its ligand specificity.

Unanswered Questions

As we explore diverse species, questions arise, prompting reflection on the human immune system.

• Why do some species have hundreds of cGLRs, while only one (cGAS) has been fully characterized for humans? Future exploration should extend to the Mab-21 Family, intriguingly predicted as human NTases based on their fold, despite their notable sequence divergence. Validating their predicted function, uncovering potential ligands, and discerning their role in emerging immune responses are of interest.
• In which scenarios do these newly discovered nucleotides crossover to other species, a phenomenon limited to artificial lab settings? Can a nucleotide identified e.g. in corals, absent in the human repertoire, effectively transduce signals in the human system? If so, does its signaling pathway align with the STING cascade, or does it deviate, prompting exploration into alternative pathways for cellular communication?
• Amidst these scientific inquiries, the prospect of utilizing these nucleotides in clinical settings beckons. The notion of employing them as adjuvants for vaccines takes center stage, yet the method of delivery remains an ongoing investigation.

Conclusion

In conclusion, our exploration of the cGAS-STING signaling axis has unveiled its adaptability and diversity across species. The discovery of over 3,000 cGAS-like receptors prompts questions about their roles in different organisms and encourages us to continue the investigation of other human NTases. Insights into second messenger molecules like 2’3’-cUAMP challenge traditional beliefs, reshaping our understanding of STING activation. Lastly, these discoveries serve as a prime example of the need to occasionally shift away from a human-centric perspective to gain a deeper understanding of ourselves.