Downregulated ECHS1 and HADH-mediated fatty acid β-oxidation contributes to mitochondrial dysfunction in salivary glands of Sjögren's syndrome.

Background: Sjögren's syndrome (SS) is a common autoimmune disease characterized clinically by dry mouth and dry eyes resulting from impaired exocrine gland function. However, its precise pathogenesis remains incompletely understood. Mitochondria, as cellular energy hubs, maintain homeostasis via fatty acid β-oxidation (FAO), the tricarboxylic acid (TCA) cycle, and oxidative phosphorylation. ECHS1 and HADH are key enzymes in the mitochondria that catalyze the FAO process, playing crucial regulatory roles in cellular energy metabolic balance. This study aims to systematically investigate alterations in mitochondrial structure and function in the salivary glands of SS patients and mice and to elucidate the potential mechanisms of ECHS1 and HADH in SS-related mitochondrial dysfunction. Methods: To systematically evaluate the association between mitochondrial abnormalities and salivary gland secretory function in SS, human salivary gland samples from SS patients and non-SS controls, alongside non-obese diabetic (NOD) mice and ICR-control mice, were analyzed. Mitochondrial ultrastructure was examined via transmission electron microscopy (TEM), and functional indicators were measured. Transcriptome sequencing identified differentially expressed genes (DEGs), which were cross-referenced with MitoCarta3.0 to pinpoint mitochondrial-related DEGs (MR-DEGs). KEGG pathway enrichment analysis and GO functional annotation were performed. A protein-protein interaction (PPI) network was then constructed using the TCMNP database to identify key hub genes. Finally, Key hub genes ECHS1 and HADH were validated at both the transcriptional and protein levels using the GEO dataset (GSE40611), qPCR, and immunofluorescence. Results: The results revealed significant mitochondrial structural and functional abnormalities in both SS patients with salivary gland dysfunction and NOD mice. Compared to controls, salivary gland mitochondria in NOD mice exhibited marked ultrastructural damage, including swelling and cristae fragmentation, accompanied by increased ROS levels and decreased mitochondrial membrane potential. Transcriptomic analysis identified 81 MR-DEGs. KEGG enrichment analysis indicated these genes were significantly enriched in energy metabolism-related pathways such as Fatty acid biosynthesis, Fatty acid metabolism, and TCA cycle. GO analysis showed that MR-DEGs were primarily involved in biological processes like the fatty acid metabolic process, localized to the mitochondrial matrix, and possessed molecular functions such as ligase activity. PPI network analysis further screened seven hub genes (ECHS1, HADH, PCX, ACADM, ACACA, FASN, ACLY), among which ECHS1 and HADH exhibited the highest interaction score. Consistent results from GEO database analysis and qPCR validation demonstrated significantly downregulated mRNA expression levels of ECHS1 and HADH in the salivary glands of both SS patients and NOD mice. Immunofluorescence assays further confirmed a marked reduction in ECHS1 and HADH fluorescence intensity at the protein level in SS patient salivary gland tissues. Conclusion: These findings indicate that salivary gland secretory dysfunction in SS is closely associated with mitochondrial structural damage and functional disruption. The underlying mechanism involves abnormalities in energy metabolic pathways, including fatty acid β-oxidation, potentially related to metabolic disturbances mediated by the downregulation of the key enzymes ECHS1 and HADH. This discovery provides a new theoretical basis for elucidating the pathological mechanisms of glandular involvement in SS from the perspective of mitochondrial energy metabolism and suggests potential research directions for developing related therapeutic strategies.