Impaired mitochondrial function as a trigger for SI joint upregulation?

The autoimmune disease dermatomyositis is one of the collagenoses and
is characterized by muscular symptoms and characteristic skin florescences. There are indications that the disease is controlled by type I interferon. In a study presented at this year’s ADF Annual Meeting, a research team used multi-omics methods to analyze molecular pathways and uncover pathogenic signatures.

If too much interferon is produced, the overactivated immune system attacks healthy cells. Type I interferonopathies are a subtype of autoinflammatory disorders characterized by impaired nucleic acid metabolism and impaired self- and foreign nucleic acid recognition [1,8]. The activation of sensors of the innate immune system by its own nucleic acids is a relevant trigger for type I IFN-driven inflammation and cutaneous autoimmunity [2]. Earlier studies have shown, for example, that lupus erythematosus – also an inflammatory autoimmune disease of the connective tissue – can be induced by defects in intracellular nucleic acid metabolism [3]. Increased expression of interferon-stimulated genes (ISGs) in the peripheral blood is also referred to as an ISG signature [4].

Seahorse assay for the analysis of mitochondrial function

In the study by Steininger et al. RT-PCR and RNA sequencing revealed that fibroblasts isolated from skin biopsies of DM patients have an increased ISG signature in culture [1]. The Gene Set Enrichment Analysis** also showed a significant down-regulation of the metabolic pathways involved in oxidative phosphorylation. In particular, the gene expression of the mitochondrial components of complexes I and IV was reduced. This led the researchers to hypothesize that impaired mitochondrial function could be a relevant trigger factor for ISG upregulation in DM. To investigate a possible link between mitochondrial dysfunction and inflammation, they compared mitochondrial function between fibroblasts from DM patients and healthy controls using the Seahorse^Assay£. They found that both the basal oxygen consumption rate and ATP production in the patients’ cells were significantly reduced. This was consistent with MitoTracker Green or MitoSOX labeled fibroblasts by flow cytometry (fluorescence-activated cell sorting, FACS), which showed significant upregulation of reactive oxygen species in DM fibroblasts compared to healthy controls as a sign of mitochondrial stress [1,5].

** Gene Set Enrichment Analysis (GSEA) assesses whether a predefined set of genes exhibits statistically significant phenotypic differences [6].
^£ The “Seahorse Assay” enables the non-invasive measurement of mitochondrial respiration and glycolysis, which allows conclusions to be drawn about the preferred metabolic pathways of cell lines, protein-protein interactions or the effects of metabolic inhibitors [7].

Abundance of mtDNA in the cytosol of DM fibroblasts

To test whether endogenous mitochondrial DNA (mtDNA) could be a triggering factor for ISG induction in DM, the researchers analyzed the content of mtDNA in the cytosol of fibroblasts from patients and controls [1]. Interestingly, RT-PCR of several mitochondrial genes revealed a significantly increased abundance of mtDNA in cytosolic fractions of DM fibroblasts compared to healthy controls. To prove that the increased presence of cytosolic mtDNA may be causative for inflammation in DM, the researchers performed mtDNA depletion with 2′,3′-dideoxycytidine (ddC) over 9 days. This resulted in a significant downregulation of ISGs in DM fibroblasts, while maintaining the normal ISG response to G3-YSD (DNA) or pily I:C (RNA) used as viral mimicry. These data indicate that mtDNA is a triggering factor for ISG upregulation in DM fibroblasts.

Normalization of the IFN signature in STING restriction

si-RNA-mediated downregulation of the stimulator of interferon genes (STING) significantly reduced ISG expression in the patients’ fibroblasts. Given the essential role of STING in the cytosolic DNA recognition pathway, these data establish a link between mitochondrial damage and mtDNA release and the cell’s own ISG induction in DM fibroblasts. This chronic activation of the type I IFN signaling pathway can increase and maintain autoimmunity. The normalization of the IFN signature when STING is restricted suggests a new therapeutic intervention strategy for DM in cases that do not respond to conventional treatment, the study authors conclude [1].

Congress: ADF Annual Meeting

Literature:

1. Steininger J, et al.: Impact of mitochondrial stress in the pathogenesis of dermatomyositis, P161. 50^th Annual Meeting of the Arbeitsgemeinschaft Dermatologische Forschung (ADF). Exp Dermatol 2024 Mar; 33(3): e14994.
2. Schlüssel zur fälschlichen Aktivierung des Immunsystems», www.medfak.uni-bonn.de/de/fakultaet/nachrichten/schluessel-zur-faelschlichen-aktivierung-des-immunsystems, (last accessed03.06.2024)
3. Günther C: Forschung in der Praxis: Störung des intrazellulären Nukleinsäuremetabolismus fördert die Entwicklung des Lupus erythematodes. JDDG 2021; 19(2): 209–214.
4. Junt T, Barchet W: Translating nucleic acid-sensing pathways into therapies. Nat Rev Immunol 2015; 15(9): 529–544.
5. Neikirk K, et al.: MitoTracker: A useful tool in need of better alternatives. Eur J Cell Biol 2023 Dec; 102(4): 151371.
6. Subramanian AP, et al.: Gene set enrichment analysis: a knowledge-based approach for interpreting genome-wide expression profiles. Proc Natl Acad Sci USA 2005; 102(43): 15545–15550.
7. «Good Method to Study Mitochondrial Respiration», www.labcompare.com/2687-Product-Reviews/
   610561-Good-method-to-study-mitochondrial-respiration, (last accessed 03.06.2024) .
8. Hauck F: DOI: https://doi.org/10.47184/ti.2021.03.04

DERMATOLOGIE PRAXIS 2024; 34(3): 42 (published at 17.6.24, ahead of print)