Stretches of nuclear DNA can adopt a broad range of transient structural forms, guided by the complex feedback loops of epigenetic marks, decorating molecules added to and removed from DNA. Epigenetics as a form of control over gene expression is all a matter of structure: which regions are packaged up and inaccessible, which are accessible. Other processes can also have their effects on local structure of the DNA, however. Some of the resulting structural forms can be problematic, and might be thought of as damage or dysfunction or an unwanted side-effect of those processes operating on DNA. This is all fairly well described by the scientific community, with an established nomenclature for different structural features.
In this context, an R-loop is a structure in which a length of double-stranded nuclear DNA has a RNA sequence stuck to it, perhaps the consequence of a failure of transcription, the first step of gene expression. In today’s open access paper, researchers provide evidence for R-loops to result in leakage of nuclear DNA fragments from the nucleus into the cytosol. This triggers inflammatory signaling via the cGAS/STING system that evolved to detect inappropriately localized nucleic acids, such as that belonging to viruses and bacteria. Unfortunately, forms of cell damage related to aging and disease will result in mislocalized fragments of the cell’s own nucleic acids, generating a maladaptive inflammatory reaction on the part of cGAS/STING that further contributes to the progression of aging and disease.
Intervertebral disc degeneration (IVDD) is a significant contributor to chronic low back pain and disability worldwide, yet effective treatment options remain limited. Through integrative analysis of single-cell RNA-seq data from intervertebral discs (IVDs), we have firstly uncovered that the aberrant accumulation of R-Loops – a type of triple-stranded nucleic acid structure – can result in the cytoplasmic accumulation of double-stranded DNA (dsDNA) and activate cGAS/STING signaling and induce cellular senescence in nucleus pulposus cells (NPCs) during IVDD. Restoring the R-Loop state significantly mitigated both the activation of the cGAS/STING pathway and NPC senescence. Additionally, we identified ERCC5 as a critical regulator of the R-Loop state and cellular senescence.
Thus, we developed an NPC-targeting nano-delivery platform (CTP-PEG-PAMAM) to deliver small interfering RNA for ERCC5 (si-Ercc5) to the NP region of the IVDD. This approach aims to modulate the abnormal R-Loop state and inhibit the activation of cGAS/STING signaling in NPCs for IVDD treatment. CTP-PEG-PAMAM demonstrated excellent targeting capability towards NPCs and NP tissue, and achieved effective silencing of the Ercc5 gene without causing systemic organ complications. Both in vitro and in vivo experiments revealed that CTP-PEG-PAMAM-siERCC5 significantly inhibited cGAS/STING signaling activated by aberrant R-Loops, alleviated cellular senescence and promoting cell proliferation, thereby delayed IVDD in a puncture-induced rat model.
In conclusion, the ERCC5-R-Loop-cGAS/STING axis in NPCs represents a promising therapeutic target for delaying IVDD, and the designed CTP-PEG-PAMAM/siRNA complex holds great potential for clinical application in the treatment of IVDD.
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